CardsplainThe fine print
What counts: each land is its own trigger, so when Through the Forest Gate puts four lands onto the battlefield at once, Thranduil, Sindarin Liege makes four Elf . A land you discard to Silvan Reveler goes onto the battlefield, so it counts too. A land leaving does nothing, so sacrificing Elvenking's Halls gives no trigger.
Any turn: the ability names no turn. You play lands only on your own turn, but a Hobbit Hole or Elven Passage cracked on the opponent's turn triggers too. That is how Attercop gets its +1/+1 while blocking.
New creatures: landfall works on a creature that entered this turn. Thranduil's Company can put two +1/+1 counters on the Wood Elves whose Forest triggered it. The it gives doesn't remove , so Wood Elves still can't attack that turn, and neither can an Elf token made this turn.
Until end of turn: Attercop's +1/+1 and Boughside Wanderers' +2/+2 last until the cleanup step, the last step of the turn. Damage is removed at the same moment, so a pumped creature with damage doesn't die when the bonus ends.
The : playing a land doesn't use the stack, so nobody can respond to it, and Mirkwood Pathmaker counts the land before any landfall trigger is on the stack. The trigger does use the stack. If the opponent kills the creature Thranduil's Company targets, the counters never come. If they kill Thranduil, his trigger still makes the token, as a 1/1. When one land triggers several abilities you choose their order, but each picks its target as it goes on the stack, so Thranduil's Company can't target the token that the same land is about to make.
Shrinking: from the moment you Hobbit Hole until the basic land arrives, Pathmaker counts one land fewer, so a 4/4 is a 3/3, and damage on it can kill it in that gap.