1. According to sandy55, M spring/hammer assemblies are interchangeable.
2. + 3. You'll have to go as far as necessary for a
repair, so it's not entirely trivial. Environment should be as clean as possible when handling membranes.
4. One would hope so. Possibly the issue can already be fixed by performing the stud replacement.
5. There were
a number of board revisions for the full-sized M alone, not all of which are interchangeable, so I guess you'd have to have a donor M of the correct vintage at the very least. However, there's not terribly much to break on there. Maybe you'd have to swap the two electrolytics one of those days, but that's about it.