Stack Master

If my [[Vampire Nighthawk]] and my opponent's creature trade in combat, when do I gain the life from lifelink? If I have [[Future Sight]] and play a [[Hallowed Fountain]] from the top of my library, do I get to see the next card of my library before I decide whether to pay 2 life to have it enter untapped? If my red creature is turned into a green Elk by [[Oko, Thief of Crowns]], does my devotion to red decrease or my devotion to green increase? If I cast [[Explore the Underdark]] to search for [[Gond Gate]] and [[Manor Gate]], do they enter the battlefield tapped or untapped? I control [[Grizzly Bears]] and [[Silent Arbiter]], and my opponent controls [[Season of the Witch]]. If I attack with only the [[Silent Arbiter]], are my [[Grizzly Bears]] destroyed at the end of the turn?