In general, to reveal a card means only to "show that card to all players for a brief time", that is, "for as long as necessary to complete the parts of the effect that card is relevant to" (C.R. 701.16a). This is the case, for example, with Nissa, Vastwood Seer and Nissa, Sage Animist.
There are some effects that require players to reveal cards continuously, such as found in Courser of Kruphix or Sen Triplets; they have the form "play with [certain cards] revealed"; in general, cards have to be revealed for as long as such an effect is active and they meet that effect's criteria (C.R. 611.2a, 611.3b).
There are also spells that include revealing a card in their costs (e.g., Silvergill Adept), activated abilities that include revealing a card in their activation costs (e.g., ninjutsu [C.R. 702.48a]), and triggered abilities that trigger when a card is revealed (such as found in Rowen or Keranos, God of Storms). For such spells and abilities, the card in question is revealed until the spell or ability leaves the stack (or if the triggered ability isn't put on the stack at the appropriate time) (C.R. 701.16a).
EDIT (Sep. 6, 2020): Edited, including because one rule was renumbered in the meantime.
Would it prevent noncombat damage pre Dominaria? Since at that time you would do damage to a player and than redirect it to a planeswalker?
Under the rules that were in effect just before Dominaria, if noncombat damage would be dealt to a player who controlled Glacial Chasm and a planeswalker by a source controlled by one of the player's opponents, there would then be two effects that would change what would happen to that damage, namely the Glacial Chasm effect and the planeswalker redirection effect (C.R. 306.7, as was in effect in Rivals of Ixalan). The player who would be dealt damage would choose which effect applied, and the other would do nothing since damage would no longer be dealt to that player (C.R. 616.1, 616.1d, 616.1e, as were in effect in Rivals of Ixalan).
All Is Dust is not a creature spell, so the mana you add due to Cavern of Souls's last ability can't be spent at all to cast All Is Dust, as opposed to the mana you add due to Cavern of Souls's second ability ("T: Add C"), mana that has no restriction or effect attached to it.
For the purposes of Astral Drift, to cycle a card means to discard that card "to pay an activation cost of a cycling ability" (C.R. 702.28c). Thus, cycling a card necessarily happens while paying an activation cost of a cycling ability, and by extension while activating a cycling ability (C.R. 602.2b, 601.2h). And activating a cycling ability necessarily uses the stack, since cycling abilities don't produce mana and so are not mana abilities (C.R. 702.28a, 702.28e-f, 602.2a, 605.1a, 605.3b). (Note that if a spell with split second is on the stack, players can't activate abilities other than mana abilities, so that players can't activate cycling abilities since, again, they're not mana abilities [C.R. 702.60a, 702.28a, 702.28e-f, 605.1a].)
In general, if you control Bolas's Citadel, you may look at the top card of your library while a spell or ability is resolving, among other moments. The second ruling you cite mentions a limited exception that applies when "the top card of your library changes while you're casting a spell ... or activating an ability" (a notable example is drawing a card due to Chromatic Sphere while casting a spell) (C.R. 401.5), which can't occur while Tainted Pact is resolving. See also this thread.
The rules update for Modern Horizons added C.R. 706.9e. Due to this new rule, if a replacement effect of the form "enter the battlefield as a copy of [an object], except it enters with ... additional +1/+1 counter[s] on it", the additional +1/+1 counters are not added on if "another copy effect is applied to" the permanent entering the battlefield this way. Here is an example that illustrates this rule.
One Spark Double (Spark Double I) is on the battlefield and isn't a copy of anything. You cast another Spark Double (Spark Double II).
All players pass, then Spark Double II resolves. You have Spark Double II become a copy of Spark Double I. Spark Double II thus acquires Spark Double I's copiable values and would enter the battlefield with one additional +1/+1 counter on it.
Now, the effect from Spark Double I's second ability is applicable (C.R. 616.1e, 616.2). You have Spark Double II again become a copy of Spark Double I. Thus, Spark Double II would now enter the battlefield with only one additional +1/+1 counter on it (not two) (the part of the previous replacement effect that would add counters to Spark Double II doesn't happen anymore) (C.R. 706.9e).
Now, the effect from Spark Double I's second ability is again applicable (C.R. 616.1e, 616.2). You have Spark Double II again become a copy of Spark Double I. Thus, Spark Double II would now enter the battlefield with only one additional +1/+1 counter on it (not two and not three) (the part of the previous replacement effects that would add counters to Spark Double II doesn't happen anymore) (C.R. 706.9e).
This forms a loop that ends, for example, when you choose not to have Spark Double II become a copy of anything.
And here is another example:
One Spark Double (Spark Double I) is on the battlefield and isn't a copy of anything, and one Clone is also on the battlefield and isn't a copy of anything. You cast another Spark Double (Spark Double II).
All players pass, then Spark Double II resolves. You have Spark Double II become a copy of Spark Double I. Spark Double II thus acquires Spark Double I's copiable values and would enter the battlefield with one additional +1/+1 counter on it.
Now, the effect from Spark Double I's second ability is applicable (C.R. 616.1e, 616.2). You have Spark Double II become a copy of Clone. Thus, due to the Spark Double I effect, Spark Double II would now enter the battlefield with only one additional +1/+1 counter on it (not two) (the part of the previous replacement effect that would add a counter to Spark Double II doesn't happen anymore) (C.R. 706.9e).
Now, the effect from Clone's ability is applicable (C.R. 616.1e, 616.2). You have Spark Double II again become a copy of Spark Double I. Thus, due to the Clone effect, Spark Double II would now enter the battlefield with no additional +1/+1 counter on it (and not with one or with two) (the part of the previous replacement effects that would add counters to Spark Double II doesn't happen anymore) (C.R. 706.9e).
This forms a loop that ends, for example, when you choose not to have Spark Double II become a copy of anything.
Mycosynth Lattice's second ability, among other things, makes all "cards that aren't on the battlefield" colorless, including cards in libraries, hands, and graveyards (C.R. 400.1), but cards outside the game are not made colorless this way (for example, a player can still choose a multicolored card they own from outside the game due to Glittering Wish) (C.R. 400.10c; see also this thread).
Changeling's effect is a type-changing effect, which applies in an earlier layer (C.R. 613.1d, layer 4; C.R. 702.72a) than effects that remove abilities (such as found in Kasmina's Transmutation) (C.R. 613.1f, layer 6). Because of this, the effect of changeling on the creature enchanted by Kasmina's Transmutation (if it otherwise has changeling) will still give the creature all creature types even though that creature loses changeling due to Kasmina's Transmutation. See also this thread.
Nether Void's ability doesn't trigger as any player's upkeep begins, nor does it affect creatures on the battlefield (C.R. 108.1).
If you are confusing that ability with that of Braids, Cabal Minion, note that the latter ability doesn't target anything (C.R. 115.1d), so that when that ability resolves, the appropriate player can choose even a creature with hexproof to sacrifice (and must sacrifice that creature if they control no other creatures, no other artifacts, and no other lands) (review C.R. 702.11b).
EDIT (Oct. 29): Edit citation of one rule in view of Core Set 2020 update.
None of Melira's three abilities affect effects that say "get[s] -1/-1", which is not the same as putting a -1/-1 counter on any permanent (compare C.R. 613.3c, which applies to such an effect, with C.R. 613.3d; see also this thread).
Melira's third ability applies to creatures "your opponents control"; it doesn't apply to any other permanent or to any other source controlled by opponents, even if the permanent or source has infect.
Not necessarily? I don't understand. Could you give me an example of a situation in which I a) would and b) would not shuffle when a Clone of an Ulamog dies?
I was thinking of cases when a permanent that is not normally Ulamog becomes a copy of Ulamog and then dies:
a. If a permanent that's normally an Emrakul, the Aeons Torn card somehow becomes a copy of Ulamog and then dies, then the object that enters the graveyard this way is Emrakul, with Emrakul's last ability which is of the form "When[ever] [this object] is put into a graveyard from anywhere, ...", so that ability will trigger when Emrakul enters the graveyard this way.
b. If a permanent that's normally a Clone card (or a Grizzly Bears card) somehow becomes a copy of Ulamog and then dies, then the object that enters the graveyard this way is Clone (or Grizzly Bears, respectively), without an ability of the form "When[ever] [this object] is put into a graveyard from anywhere, ..." to trigger.
Ulamog's last ability is not a leaves-the-battlefield ability, even if it triggers when the object with it goes to the graveyard from the battlefield, because its trigger condition says "from anywhere" (C.R. 603.6c). Since it's not a leaves-the-battlefield ability and it's not otherwise an exception mentioned in C.R. 603.10 and subrules, what is relevant for the purposes of Ulamog's last ability is the state of the "objects that exist immediately after" the object in question goes to a graveyard from anywhere (C.R. 603.10). Therefore, if the object that entered the graveyard has Ulamog's last ability, that ability will trigger from the graveyard (C.R. 603.2). For clarity, read "When Ulamog, the Infinite Gyre is put into a graveyard from anywhere..." as "When Ulamog, the Infinite Gyre enters a graveyard from anywhere..."
Therefore:
1. I assume here that you're speaking of a "token that's a copy" of Ulamog. A token that's a copy of Ulamog has entered the graveyard, and it has Ulamog's last ability (which doesn't care whether that object is a card or a token), so that ability will trigger. Note that a token can go to the graveyard or anywhere else like any other permanent (C.R. 111.6; see also C.R. 111.8 and this thread). Note also that if a token that's a copy of another object is created, the token's original values will be that other object's copiable values (C.R. 707.2, 707.3, 613.1; see also C.R. 707.5). This is different from a copy effect discussed in question 2 (compare C.R. 613.1 with 613.1a).
2. Not necessarily.Assuming you mean a permanent that's normally a Clone card but enters the battlefield as a copy of Ulamog, no. Clone's copy effect applies only to the permanent that's a copy of Ulamog due to that effect (see also C.R. 611.2c); it doesn't apply to the object that entered the graveyard as given in your question (with the assumption just given), so that that object will not necessarily have Ulamog's last ability.
3. Sudden Spoiling's effect applies only to Ulamog on the battlefield (C.R. 611.2c), not to the object that entered the graveyard as given in your question, so that the object that enters the graveyard this way will have all its usual abilities. In the case of Ulamog, therefore, its last ability will trigger when it enters the graveyard this way.
EDIT: Edited after comment 4 was posted.
EDIT (Jul. 11): Some rules were renumbered with Core Set 2020. Correctness edit.
EDIT (Aug. 13, 2020): Clarification.
EDIT (Oct. 16, 2021): Delete rule citation. Some rules were renumbered in the meantime.
Entwine expresses an additional cost (C.R. 702.41a). And effects that let you cast a spell by paying a different cost "rather than pay[ing] its mana cost", as in Bolas's Citadel, don't affect additional costs such as entwine (C.R. 117.9, 117.9d). Thus, in general, if you cast Tooth and Nail from your library with the help of Bolas's Citadel—
and choose one of that spell's modes, you pay 7 life (C.R. 608.2f, 202.3).
and choose both of that spell's modes, you pay 2, which is Tooth and Nail's entwine cost, and 7 life (C.R. 608.2f, 202.3, 702.41a).
1. Faerie Miscreant's enters-the battlefield-ability checks whether "you control" a creature other than Faerie Miscreant that is "named Faerie Miscreant"—
when Faerie Miscreant enters the battlefield (if "you don't", the ability won't trigger) (C.R. 603.4), and
when that ability would resolve (if "you don't", the ability won't resolve and you won't draw a card) (C.R. 608.2a).
2. You don't choose which creature you control other than Quickling, if any, to return to its owner's hand with Quickling's last ability until that ability resolves (C.R. 603.5, 118.12a). In general, however, if you choose such a creature in advance, you will be bound to that choice unless another player intervenes (C.R. 722.2a-c; for sanctioned tournaments, see M.T.R. 4.2). See also this thread and this thread.
EDIT (Jan. 24, 2020): Some rules were renumbered with Core Set 2020.
EDIT (Jun. 4, 2020): Edited, including because one rule was renumbered with Ikoria: Lair of Behemoths.
In this scenario, as soon as an otherwise 1/1 creature you control becomes an attacking creature, it will immediately get +1/+0 (C.R. 613.1g, 613.3c, 613.4, 611.3b, 113.6), so will become 2/1, so that Cavalcade of Calamity's ability won't trigger for that creature (C.R. 508.1k, 508.1m). (It won't trigger this way even if the game understands abilities "that trigger on attackers being declared" to trigger simultaneously on those attackers being declared [C.R. 603.10; see also C.R. 603.2].)
EDIT (Feb. 19, 2020): One rule was renumbered with Core Set 2020. Corrected another rule citation.
There are some effects that require players to reveal cards continuously, such as found in Courser of Kruphix or Sen Triplets; they have the form "play with [certain cards] revealed"; in general, cards have to be revealed for as long as such an effect is active and they meet that effect's criteria (C.R. 611.2a, 611.3b).
There are also spells that include revealing a card in their costs (e.g., Silvergill Adept), activated abilities that include revealing a card in their activation costs (e.g., ninjutsu [C.R. 702.48a]), and triggered abilities that trigger when a card is revealed (such as found in Rowen or Keranos, God of Storms). For such spells and abilities, the card in question is revealed until the spell or ability leaves the stack (or if the triggered ability isn't put on the stack at the appropriate time) (C.R. 701.16a).
EDIT (Sep. 6, 2020): Edited, including because one rule was renumbered in the meantime.
If you are confusing that ability with that of Braids, Cabal Minion, note that the latter ability doesn't target anything (C.R. 115.1d), so that when that ability resolves, the appropriate player can choose even a creature with hexproof to sacrifice (and must sacrifice that creature if they control no other creatures, no other artifacts, and no other lands) (review C.R. 702.11b).
EDIT (Oct. 29): Edit citation of one rule in view of Core Set 2020 update.
Melira's third ability applies to creatures "your opponents control"; it doesn't apply to any other permanent or to any other source controlled by opponents, even if the permanent or source has infect.
a. If a permanent that's normally an Emrakul, the Aeons Torn card somehow becomes a copy of Ulamog and then dies, then the object that enters the graveyard this way is Emrakul, with Emrakul's last ability which is of the form "When[ever] [this object] is put into a graveyard from anywhere, ...", so that ability will trigger when Emrakul enters the graveyard this way.
b. If a permanent that's normally a Clone card (or a Grizzly Bears card) somehow becomes a copy of Ulamog and then dies, then the object that enters the graveyard this way is Clone (or Grizzly Bears, respectively), without an ability of the form "When[ever] [this object] is put into a graveyard from anywhere, ..." to trigger.
Clarifying comment 2.
EDIT (Jul. 11): Correctness edit.
Therefore:
1. I assume here that you're speaking of a "token that's a copy" of Ulamog. A token that's a copy of Ulamog has entered the graveyard, and it has Ulamog's last ability (which doesn't care whether that object is a card or a token), so that ability will trigger. Note that a token can go to the graveyard or anywhere else like any other permanent (C.R. 111.6; see also C.R. 111.8 and this thread). Note also that if a token that's a copy of another object is created, the token's original values will be that other object's copiable values (C.R. 707.2, 707.3, 613.1; see also C.R. 707.5). This is different from a copy effect discussed in question 2 (compare C.R. 613.1 with 613.1a).
2.
Not necessarily.Assuming you mean a permanent that's normally a Clone card but enters the battlefield as a copy of Ulamog, no. Clone's copy effect applies only to the permanent that's a copy of Ulamog due to that effect (see also C.R. 611.2c); it doesn't apply to the object that entered the graveyard as given in your question (with the assumption just given), so that that object will not necessarily have Ulamog's last ability.3. Sudden Spoiling's effect applies only to Ulamog on the battlefield (C.R. 611.2c), not to the object that entered the graveyard as given in your question, so that the object that enters the graveyard this way will have all its usual abilities. In the case of Ulamog, therefore, its last ability will trigger when it enters the graveyard this way.
EDIT: Edited after comment 4 was posted.
EDIT (Jul. 11): Some rules were renumbered with Core Set 2020. Correctness edit.
EDIT (Aug. 13, 2020): Clarification.
EDIT (Oct. 16, 2021): Delete rule citation. Some rules were renumbered in the meantime.
2. You don't choose which creature you control other than Quickling, if any, to return to its owner's hand with Quickling's last ability until that ability resolves (C.R. 603.5, 118.12a). In general, however, if you choose such a creature in advance, you will be bound to that choice unless another player intervenes (C.R. 722.2a-c; for sanctioned tournaments, see M.T.R. 4.2). See also this thread and this thread.
EDIT (Jan. 24, 2020): Some rules were renumbered with Core Set 2020.
EDIT (Jun. 4, 2020): Edited, including because one rule was renumbered with Ikoria: Lair of Behemoths.
EDIT (Feb. 19, 2020): One rule was renumbered with Core Set 2020. Corrected another rule citation.