We couldn't find it in the rules, but we played that once a village is isolated you can no longer add anything to that village. Correct?
All four players at the table were beginners, but we were all REALLY impressed with this game.
Correct. Villages with stones are out of play. Any tiles (including students) that are on the village when the last bridge falls are left there for the remainder of the game. You don't want to get caught with lots of students stuck in those villages, as they're basically wasted.