Nick, in my view (a) is pretty much impossible to answer with any precision. There're factors besides just the size of the room, and besides, the only spot that necessarily has to be "filled" adequately is the one where your ears are. Maybe just leave it that the M80s can play a little more loudly with low distortion if other factors remain equal.
On (b), if you mean would you get more benefit from surround speakers or a center speaker if you couldn't get both immediately, in my view go with the QSs. The main speakers can do a good job forming a "phantom" center image if no real speaker is there. Real surround speakers are needed for a good surround effect, so they do add something extra.
On (c), that should work, although it's unclear if there's enough difference to bother. The gain of the 8060 amplifier section is probably different from that of the amplifiers in the 503, but could presumably be equalized when calibrating speaker levels.
