New monster manual compatible with 5e ?

So I am doing 5e campaign and wanted some monster book. I already have mordecainer presents monsters of the multiverse or whatever it's called. I thought about buying monster manual but I don't know which year. So I have 3 questions.

1. Would the new monster manual be mostly compatible with 5e?

2. Would there be big overlay in monsters present in new monster manual and mordecainer presents...?

3. Unrelated mostly but is champion fighter subclass much different in 5e than 5.5e?