A little late to this post but i can also say that using only regular floor screws (deeper threads) that the QS8s can be mounted right into drywall and remain quite firm.
The one problem we had awhile back was with the speaker cable running to the backside of the surrounds. Because the QS8 was tight against the wall, the plastic cable jacket ended up sticking to the paint which cracked and peeled during the removal of the surround speakers when we moved.
The rubber feet did something similar although i managed to keep the paint from peeling off the wall.
As for your system Bruce, we also prefer having the side surrounds slightly to the rear of our seating position (see
recent pics link again).
Our rear surrounds will go directly behind us in the near future.
In your situation, if you do not hang the QS8s directly behind your seating area, then your alternate idea of using direct radiating speakers aimed right from the back wall should work equally well. Another alternative is to mount something like M2s in the positions you noted on your drawing and then angle them forward. You would of course need something like the FMB to do this, but at least you could locate the speaker wires on the side walls rather than trying to run more wire to your back wall.
