Hi. Planning a 6.1 HT around M22 fronts and VP150 center. Since I DO NOT have side walls, considering a pair of M2's or M3's for rear L/R. For rear center, which would work best in this set-up: another M2 or M3, a VP100, or a QS4 or QS8? Thanks very much in advance.

Just a note, not having side walls should not delineate what speaker you buy for L/R rears.
The rest of your choices sound good. Personally my rear centre will be a VP100 later this year but i have also considered a single qs8 to match my qs8 surrounds.