CardsplainThe fine print
checks twice. Nasty Little Rabbit's ability has an "if" right after its trigger, so it looks when the beginning of combat step starts on your turn and again as the ability . With no 4-power creature at the start of combat it doesn't trigger at all, and a creature that grows later in that step is too late. If the opponent kills your only 4-power creature in response, the Rabbit gets no +1/+1 counter .
Damage doesn't shrink Pathmaker, but losing a land does. Damage doesn't lower power, so a 4/4 Mirkwood Pathmaker with 3 damage on it still turns ferocious on. Pathmaker counts the lands you control, or untapped, so paying for a spell doesn't shrink it. Cracking Hobbit Hole does, for a moment. You it first and the new land arrives later, so a damaged Pathmaker can die in between.
Counters leave with the creature. A creature that leaves the battlefield and returns is a new object. A Rabbit returned to your hand by Mirkwood Nurturer, or brought back by The Mountain-king's Return, starts over as a 1/2, and the counters from Troll Negotiations are gone too.
Targets are checked again on resolution. If your creature is gone when Quarrel or Troll Negotiations resolves, no damage is dealt. If only their creature is gone or has gained , Troll Negotiations still puts its on your creature, but there is no fight.
Tokens don't come back. A that Celebrate the Mountain-king exiles ceases to exist and stays gone after the leaves. If the enchantment leaves before its trigger resolves, nothing is exiled at all.
Blocked stays blocked. Killing a blocker with Quarrel after blocks doesn't let your attacker hit the player unless it has , like Large Bear.
beats . A creature dealt deathtouch damage is destroyed only when the game checks, and Stone by Sunlight's indestructible stops that. In the cleanup step the damage wears off at the same moment the indestructible does.