only the latest one applies. the game would be nigh unwinnable if you had to stack midnight effects...
Paul is correct, it's all of them, regardless of when they were dealt, and if anyone is standing on them.
Where did you get that last bit from, about someone having to be on the Adventure? All I can find is:
"If the active player advances the Clock to XII (midnight) (or if he
advanced the Clock to midnight at any point during his turn), he
now resolves all game effects in play that say "At Midnight."" (p. 9)
and that's how I've been playing them, all resolve, whether someone is on the Adventure or not.