Yes, they are interchangeable.

Yes, you can use both of them! When talking about an especially artistic movie, or analyzing movies in an academic context, I would use "film". For example, people who want to make movies professionally go to film school for university.

Yes you can, they mean the same thing

Yes. I would use both in one paragraph just for variation.

Movie is American English. Film is British English and some other languages also use it. It’s better if you use only one in the same situation.