Welcome to the forum future member of the M80s's club!
1) I think the M22s will be plenty for your size room. The M3s and M2s should also do fine if you're talking about volume levels. What king of music do you listen to? Type of movies?
2) Can't help you much even though one of the reasons to go bigger would be to have a lower extension which is nice for movies and games.
3) If the only thing that you need for now is just to power this 2 speakers and will upgrade your receiver later, I would by the cheapest I could fine, IMO any receiver should drive the M22s fine.
4) For 5.1 you would be better off with the QS series which I hear are amazing. Direct radiating speakers just don't work as well. If you would be considering a possibility of 7.1 in the future, I would probably get the M2s for now and then later on use them as rears. If not, any of these choices should be fine and you could always use them as a second stereo system.
