If an enchantment enters the battlefield while Opalescence is on the battlefield, that enchantment is a creature from the moment it starts being a permanent. It's never a noncreature permanent while Opalescence is on the battlefield. Thus, triggered abilities that trigger when a creature enters the battlefield will trigger.
Creature tokens are still creatures, and creating a token on the battlefield will trigger abilities that trigger when a creature enters the battlefield.
You control the creature, your opponent controls the Aura. (If this is confusing, it may help to represent who controls what by rotating the cards so that they face their controller.) The Aura will remain attached to that creature after you gain control of it and it will still grant the bonus, calculating the bonus based on the number of plains controlled by the controller of the Aura, since that's what "you" refers to.
If (say) Lightning Bolt is resolving targeting you while you control a planeswalker, and there's an effect from Intervention Pact waiting to prevent damage, you get to choose which effect will redirect/prevent the damage, since both effects (the built-in planeswalker redirection and Intervention Pact) are attempting to modify the same damage.
If you choose Intervention Pact, then Lightning Bolt's damage is prevented and you gain 3 life.
If you choose the planeswalker redirection effect, then your opponent has the option to deal damage to a planeswalker you control. If they opt to redirect, that's where the damage will go. If they opt not to redirect, Intervention Pact's effect will still prevent the damage and you'll gain 3 life.
Intervention Pact, as well as other effects that prevent damage to players, have no interaction with combat damage dealt to a planeswalker as a result of that planeswalker being attacked because the damage is never assigned to you.
Targeting Precursor Golem with a kicked Rite will trigger the Golem. You'll get three kicked Rites each targeting a different Golem at that moment. The first one to resolve will make five copies of one of the tokens, and the second will do the same. Then the original will resolve and make five copies of Precursor, each of which brings two tokens with it. You'll end up with 5 copies of Precursor Golem, 10 copies of tokens, and 10 more "original" tokens.
Phytohydra's ability replaces damage. Infect changes the result of damage. Phytohydra's ability will replace infect damage in the same way that it replaces any other damage, and it will get +1/+1 counters. The damage doesn't have the chance to put -1/-1 counters on Phytohydra since it's never actually dealt.
810.9. Damage, loss of life, and gaining life happen to each player individually. The result is applied to the team's shared life total.
Example: In a Two-Headed Giant game, a player casts Flame Rift, which reads, "Flame Rift deals 4 damage to each player." Each team is dealt a total of 8 damage.
"Opponent" and "player" still refer to a single person playing the game. In a 2HG game, Breath of Malfegor will usually cause the opposing team's life total to go down by 10, and an Earthquake will usually cause each team's life total to go down by X times 2.
At end of turn the Creature - Elemental will be removed as a delayed triggered ability caused by Stirring Wildwood's triggered ability.
Not quite. "Until end of turn" represents a duration for a continuous effect, not a delayed triggered ability. At the cleanup step, the effect making the Stirring Wildwood a creature will simply wear off. This doesn't use the stack.
Spreading Seas will resolve first, since it's on the top of the stack. When it resolves, Stirring Wildwood will become an Island and lose its ability. Then the already-activated instance of the ability it had will resolve, and make it a creature. It'll be:
Stirring Wildwood
Land Creature - Island Elemental (T: Add to your mana pool.)
Reach
3/4
and it will be green and white.
Situation 2:
The Stirring Wildwood ability will resolve, making it a creature. Then the Spreading Seas will resolve, making it an Island, but that doesn't touch it's creature types (or the creature type). It won't lose reach, since that isn't an ability granted by its rules text. It will look exactly the same as the Stirring Wildwood in situation 1.
You can cast Silence in your opponent's draw step.
You can't cast Silence "in response" to your opponent playing a land. When a player plays a land, they retain priority. Unless they pass priority to you, they will be able to cast a spell.
You can't cast Silence "in response" to your opponent tapping a land for mana when they have priority, for the same reason as above. They'll retain priority.
You can't cast Silence "in response" to your opponent tapping a land for mana while they're casting a spell or activating another ability, since no one has priority at that time.
If Protean Hydra with two +1/+1 counters on it would be damaged by Lightning Bolt, the replacement effect generated by its second ability tries to remove three counters from it instead. Since there are only two, the effect does as much as it can, and removes both. This causes its third ability to trigger twice.
State-based effects are checked, and the Hydra is put into its owner's graveyard for having zero toughness. Both its triggers are put on the stack, and each will set up a delayed triggered ability.
At the beginning of the next end step, the two delayed triggered abilities will go on the stack. When they resolve, the Protean Hydra is long gone, and they will do nothing.
The turn consists of five phases, three of which are broken into steps. They proceed as follows:
Beginning phase
Untap step
Upkeep step
Draw step
Precombat main phase
Combat phase
Beginning of combat step
Declare attackers step
Declare blockers step
Combat damage step
End of combat step
Postcombat main phase
Ending phase
End step
Cleanup step
During most of these steps (including the end step), players get priority to cast spells and activate abilities. If the Protean Hydra loses a +1/+1 counter during any phase that isn't the ending phase, then the "beginning of the next end step" will be during the current turn. If the Protean Hydra loses a +1/+1 counter during the ending phase, then its end-step trigger will not trigger until the beginning of the end step of the next turn, since the game has already moved through the beginning of the end step of the current turn.
Here's the rules entry for the End step:
Quote from "Comprehensive Rules" »
513. End Step
513.1. First, all abilities that trigger "at the beginning of the end step" or "at the beginning of the next end step" go on the stack. (See rule 603, "Handling Triggered Abilities.")
513.1a Previously, abilities that trigger at the beginning of the end step were printed with the trigger condition "at end of turn." Cards that were printed with that text have received errata in the Oracle card reference to say "at the beginning of the end step" or "at the beginning of the next end step."
513.2. Second, the active player gets priority. Players may cast spells and activate abilities.
513.3. If a permanent with an ability that triggers "at the beginning of the end step" enters the battlefield during this step, that ability won't trigger until the next turn's end step. Likewise, if a delayed triggered ability that triggers "at the beginning of the next end step" is created during this step, that ability won't trigger until the next turn's end step. In other words, the step doesn't "back up" so those abilities can go on the stack. This rule applies only to triggered abilities; it doesn't apply to continuous effects whose durations say "until end of turn" or "this turn." (See rule 514, "Cleanup Step.")
(Also note entry 513.1a -- "At end of turn" has been errata'd.)
If an enchantment enters the battlefield while Opalescence is on the battlefield, that enchantment is a creature from the moment it starts being a permanent. It's never a noncreature permanent while Opalescence is on the battlefield. Thus, triggered abilities that trigger when a creature enters the battlefield will trigger.
Creature tokens are still creatures, and creating a token on the battlefield will trigger abilities that trigger when a creature enters the battlefield.
If you choose Intervention Pact, then Lightning Bolt's damage is prevented and you gain 3 life.
If you choose the planeswalker redirection effect, then your opponent has the option to deal damage to a planeswalker you control. If they opt to redirect, that's where the damage will go. If they opt not to redirect, Intervention Pact's effect will still prevent the damage and you'll gain 3 life.
Intervention Pact, as well as other effects that prevent damage to players, have no interaction with combat damage dealt to a planeswalker as a result of that planeswalker being attacked because the damage is never assigned to you.
That's true. This case is covered by the rule quoted above, and the printed wording is likely an oversight on Wizards' part.
However, they've updated the text of the card in the Oracle to be more clear on where the cards should go.
Not quite. "Until end of turn" represents a duration for a continuous effect, not a delayed triggered ability. At the cleanup step, the effect making the Stirring Wildwood a creature will simply wear off. This doesn't use the stack.
Spreading Seas will resolve first, since it's on the top of the stack. When it resolves, Stirring Wildwood will become an Island and lose its ability. Then the already-activated instance of the ability it had will resolve, and make it a creature. It'll be:
Stirring Wildwood
Land Creature - Island Elemental
(T: Add
Reach
3/4
and it will be green and white.
Situation 2:
The Stirring Wildwood ability will resolve, making it a creature. Then the Spreading Seas will resolve, making it an Island, but that doesn't touch it's creature types (or the creature type). It won't lose reach, since that isn't an ability granted by its rules text. It will look exactly the same as the Stirring Wildwood in situation 1.
You can't cast Silence "in response" to your opponent playing a land. When a player plays a land, they retain priority. Unless they pass priority to you, they will be able to cast a spell.
You can't cast Silence "in response" to your opponent tapping a land for mana when they have priority, for the same reason as above. They'll retain priority.
You can't cast Silence "in response" to your opponent tapping a land for mana while they're casting a spell or activating another ability, since no one has priority at that time.
State-based effects are checked, and the Hydra is put into its owner's graveyard for having zero toughness. Both its triggers are put on the stack, and each will set up a delayed triggered ability.
At the beginning of the next end step, the two delayed triggered abilities will go on the stack. When they resolve, the Protean Hydra is long gone, and they will do nothing.
Short story: Lightning Bolt kills the Hydra.
Here's the rules entry for the End step:
(Also note entry 513.1a -- "At end of turn" has been errata'd.)