Welcome, Dave. Although the M3 is a very fine speaker, I wouldn't proceed as you're planning to do. Placing M3s on stands to the side is adequate, but the broader dispersion of the QS4s is an advantage for most types of surround material(not on DVD-As/SACDs if a performer is specifically placed in a surround channel). The QS4s could be mounted on the back wall and widely separated, almost to where a side wall would have been.
The 2400 is an excellent choice, and certainly does allow use of the sub with 2-channel material. Unless a mode which bypasses bass processing is selected you set the M22s small, SWFR, and crossover to it preferably at 80Hz; no bass is lost. Incidentally, my M22s give usable response down to 40Hz with room reinforcement. The specs are measured under anechoic conditions.
As I frequently do, I'll also suggest that you consider a vertical M2 for the center speaker, which I prefer to a horizontal configuration because of better horizontal dispersion.
