Abilities that trigger when a creature dies see the creature as it existed on the battlefield, but if the ability takes an action on the card in the graveyard, at that point the card is a different game object with no relation to what it looked like on the battlefield. So a combined creature where one of the creatures had undying would only return that one creature card when the undying ability resolves.
If both creature cards have undying, then the combined creature has two instances of undying, each of which refers to the creature card it's on. So they would both trigger and return their respective cards.
3BR, T: Put two target creature cards from graveyards onto the battlefield combined into one creature under your control.
There are two number words there. Increasing or decreasing the first is apparent enough in what it would do (I would rule that if you combine one creature, it's still combined) but how do you combine two creature cards into two creatures? Or five, if you have enough copies of More or Less? Which one(s) are represented by the physical card(s)?
(I expect the answer is "we don't know yet" but I'd be curious to know if there ends up being an actual ruling on this.)
The Cage example is a little bit subjective from a judging perspective. Actually putting Dryad Arbor onto the battlefield is a Game Rule Violation, subject to a warning and either leaving the game state as is or rewinding, depending on how disruptive a rewind would be. But I wouldn't issue the infraction if it's clear that the Arbor player had just revealed the card and could plausibly still be reviewing the board state.
Fetching a Dryad Arbor into Containment Priest is a much clearer case, because it's a completely legal play (if a bad one.) If neither player noticed the Priest and allowed the Arbor to be on the field for a while then we're still in GRV/FTMGS territory. But the Priest player is free (and in fact obligated, if the Arbor player has clearly finalized their selection) to just declare that the effect applied and Arbor is exiled.
The semantics of my hypothetical may have been off, but I think we agree on the basic point I was trying to make, which is that there isn't a state where you're somehow stuck with your selection of Dryad Arbor and can't put it onto the battlefield. The option exists to pick something else.
If you actually try to put Dryad Arbor onto the battlefield and finish resolving the ability, then that's a Game Rule Violation.
But before that: Putting Dryad Arbor onto the battlfield is an impossible action, so you simply can't do it. Note that Wooded Foothills just has you search and then put a chosen card onto the field - not, say, search, pick a card, reveal it, and then put it onto the field. The selection of the card and the zone change are essentially the same action in this case, so by extension you can't pick Dryad Arbor from your search. If you're in the middle of resolving the effect when you realize the Cage is out, then your options are to choose a different Forest/Mountain or fail to find.
Assuming you didn't choose red or green for Apostle's Blessing, it will resolve first and all of the other instants will resolve in reverse order. Note that in this scenario, Temur Battle Rage can't see the pump from the Mutagenic Growths when it resolves to determine whether or not you get the ferocious bonus.
This assumes that the player is casting all of the instants in response to each other such that they all end up on the stack at once. Generally it is assumed (because it is usually the better option tactically) that a player who is casting multiple instants (or cards with flash, or activating multiple abilities, or some combination) does them one at a time and allows each to resolve before casting the next, unless they specify otherwise.
By doing it this way, you can get the boosts from all of them and then be able to choose red or green for Apostle's Blessing's effect, as well has have the flexibility to respond to an opponent's responses (such as them trying to Shock the creature in response to the first Mutagenic Growth.)
In the interest of not leaving you empty-handed on the last question (since "what can stop counterspells" is a rules question of a sort if the question is general enough), there are generally two types of effects that can stop a counterspell, those being:
1. Your own counterspells, and
2. Effects that make it so that the spell you are casting can't be countered. These are usually either static abilities (e.g. Gaea's Herald) or the result of a one-shot effect (such as Autumn's Veil or Vexing Shusher.)
Note that if you make use of both Lightning Runners' abilities during the same combat, you will get both untap effects for your creatures right away during that first combat, and then two extra combat phases are created. Nothing else will untap your creatures in between those extra combat phases, so unless you're specifically trying to get extra hits in with a creature that has vigilance or you have another way to untap things, it's usually best to use one ability, let the next combat phase happen, and use the ability again. That way, the second untap happens after you've attacked for the second time, allowing a third attack.
That all looks correct as stated. The one nitpick I have is that the last instance of the "untap this creature" ability won't untap Merieke because it's already untapped (untapping an already untapped creature is an impossible action) so the ability will just resolve and do nothing.
Is there a reason why your opponent thinks it doesn't work? I suspect it might have something to do with the part where you tap Merieke in response to a triggered ability that would untap it, which is perfectly legal. The triggered ability goes onto the stack regardless of whether Merieke is tapped at the time.
Its power would be 7, since two instances of its triggered ability will have resolved. Multiple instances of an effect that modifies a creature's power and toughness are cumulative (or they effectively cancel each other out, if one effect is increasing it and another is decreasing it.)
So, enchantment destruction and countermagic will kill the combo
Maybe, maybe not.
Enchantment destruction will kill the combo if they destroy Sunbird's Invocation before you cast Approach of the Second Sun, but it won't do anything if you have already gotten the Invocation to trigger (removing the source of a triggered ability doesn't affect triggered abilities on the stack.) If you can manage to produce enough mana to cast Invocation and then immediately cast Approach, your opponent can't do anything since you get priority first after Invocation resolves (assuming you're not somehow casting it on an opponent's turn, and that nothing triggers from Invocation entering the battlefield.)
Countermagic will keep you from winning the game if it's pointed at the copy you cast from your hand (Tango). Countering the Cash copy won't do much because it still counts as having been cast as far as Tango is concerned.
Rule 701.15b doesn't apply to this scenario because you're not searching a zone. Choosing a card from a revealed hand isn't the same thing as searching it. (Compare Surgical Extraction, which specifically instructs you to search a player's hand, and consequently would allow you to fail to find.) You must choose a card if you are able to.
The full procedure here when a player leaves the game is:
1. All objects owned by that player leave the game completely. (This will end the effect of, say, Control Magic if that player owns it.)
2. All continuous effects that cause that player to control things end. (This is the relevant step for Goblin Festival in this scenario.)
3. If there are any objects not owned by that player that they would still control after ending all control-changing effects, those objects are exiled. This can happen if that player was the default controller of something because it entered the battlefield under their control, e.g. via Bribery or Gather Specimens.
So my question is obviously about Library of Leng. So the version of the card I have says "Skip your Discard Phase", while others have "You have no maximum hand size". This is the same thing correct? Do they both follow the same rules?
Just as a bit of rules background on this: There isn't a "discard phase" as a distinct part of the turn anymore, which makes the original text "skip your discard phase" meaningless under the current rules framework. Players discard down to their maximum hand size during the cleanup step, but a bunch of other things happen during the cleanup step as well (notably, removing marked damage from creatures and ending continuous effects with "until end of turn" durations) so skipping that step outright would cause other problems. The change to "You have no maximum hand size" is what preserves the card's functionality.
Because the ninja is just that good. That's my theory anyway.
If both creature cards have undying, then the combined creature has two instances of undying, each of which refers to the creature card it's on. So they would both trigger and return their respective cards.
There are two number words there. Increasing or decreasing the first is apparent enough in what it would do (I would rule that if you combine one creature, it's still combined) but how do you combine two creature cards into two creatures? Or five, if you have enough copies of More or Less? Which one(s) are represented by the physical card(s)?
(I expect the answer is "we don't know yet" but I'd be curious to know if there ends up being an actual ruling on this.)
Fetching a Dryad Arbor into Containment Priest is a much clearer case, because it's a completely legal play (if a bad one.) If neither player noticed the Priest and allowed the Arbor to be on the field for a while then we're still in GRV/FTMGS territory. But the Priest player is free (and in fact obligated, if the Arbor player has clearly finalized their selection) to just declare that the effect applied and Arbor is exiled.
But before that: Putting Dryad Arbor onto the battlfield is an impossible action, so you simply can't do it. Note that Wooded Foothills just has you search and then put a chosen card onto the field - not, say, search, pick a card, reveal it, and then put it onto the field. The selection of the card and the zone change are essentially the same action in this case, so by extension you can't pick Dryad Arbor from your search. If you're in the middle of resolving the effect when you realize the Cage is out, then your options are to choose a different Forest/Mountain or fail to find.
By doing it this way, you can get the boosts from all of them and then be able to choose red or green for Apostle's Blessing's effect, as well has have the flexibility to respond to an opponent's responses (such as them trying to Shock the creature in response to the first Mutagenic Growth.)
1. Your own counterspells, and
2. Effects that make it so that the spell you are casting can't be countered. These are usually either static abilities (e.g. Gaea's Herald) or the result of a one-shot effect (such as Autumn's Veil or Vexing Shusher.)
Is there a reason why your opponent thinks it doesn't work? I suspect it might have something to do with the part where you tap Merieke in response to a triggered ability that would untap it, which is perfectly legal. The triggered ability goes onto the stack regardless of whether Merieke is tapped at the time.
Enchantment destruction will kill the combo if they destroy Sunbird's Invocation before you cast Approach of the Second Sun, but it won't do anything if you have already gotten the Invocation to trigger (removing the source of a triggered ability doesn't affect triggered abilities on the stack.) If you can manage to produce enough mana to cast Invocation and then immediately cast Approach, your opponent can't do anything since you get priority first after Invocation resolves (assuming you're not somehow casting it on an opponent's turn, and that nothing triggers from Invocation entering the battlefield.)
Countermagic will keep you from winning the game if it's pointed at the copy you cast from your hand (Tango). Countering the Cash copy won't do much because it still counts as having been cast as far as Tango is concerned.
1. All objects owned by that player leave the game completely. (This will end the effect of, say, Control Magic if that player owns it.)
2. All continuous effects that cause that player to control things end. (This is the relevant step for Goblin Festival in this scenario.)
3. If there are any objects not owned by that player that they would still control after ending all control-changing effects, those objects are exiled. This can happen if that player was the default controller of something because it entered the battlefield under their control, e.g. via Bribery or Gather Specimens.