Well, I'd imagine they'd be treated in a manner similar to the way Germany and Japan were treated after WWII.
Let them rebuild, but on a very short leash for a while, until they could be trusted to work and play well with others.
This seems logical.
The Federation would have every incentive to try and help the natives, and to form an alliance or offer membership candidacy. That would probably continue even if Cardassia decided to again go for something else than a civilian government.
Doubtful. This would alienate the Bajorans and hurt the Federation's position with regard to using the Wormhole. It would also make it more difficult to talk the Bajorans into joining the Federation at some point in the future.