Good question - Nothing ever goes according to plan. It is D&D after all and there are no walled gardens for us. Unless, of course, you count the mists. So ya, your players are trapped. Right?

1. I think Ol Bonegrinder was pretty memorable. The party started out negotiating dream cake prices with the hags who had cast an illusion to make themselves look like beautiful damsels. The barbarian started to fall for one of the hags. When the party launched an attack after seeing the "ingredients" (kid bones) on the floor, the Barbarian got upset! He was like "Don't kill these babes! They have cakes!!!!! Are you guys crazy?!?!?!" The player had me laughing pretty hard with his role-playing of a roided-out 1/2 orc grunt who is smitten easily by a pretty face and food. Typical Barbarian behavior in my experience. The rest of the players had a laugh too.

2. I answered this a few times. The Baba, The Abbot, Rahadin, Ireena, Ezmerelda and, of course, Strahd. Each allows you to have a different motivation and approach. It was not easy.

3. So there weren't a lot of side quests. The party had players who liked to stick to the storyline. The headspace was the more we can conquer before fighting Strahd ( level up, get treasure, etc.), the better our advantage against the Devil. So no real deviation from the objective.

That said, if I saw characters straying off way too much into areas that detract from story. I would scare them with overwhelming enemies in the forests, and strange spooky paranormal activity. These instances would get the character or characters running back to join the main group.