Yes they are but they may not be compatible with all existing code as it is presently written. For example turning on OverloadedStrings can introduce type ambiguity in existing code where the type of a string literal cannot be deduced.

No, they are not, and some are useless

Edit: I guess not useless, but over time rendered as anti-features

Can you name some extensions that are not compatible with each other. I can't, off the top of my head, and I've been using Haskell for some years. (I don't deny there might be some, but I'm finding it hard to think of any.)

GeneralizedNewtypeDeriving and DeriveAnyClass (although this is fixed in 8.2)