Tob, the QS8s are still side surrounds in a 5.1 setup even if they're mounted on the back wall. For best results they should be positioned directly to the side of the listening position or slightly(maybe 2')farther back. Repositioning the QS8s should be done.
If you want to use the 3.1s, they can be positioned as back surrounds in 7.1. Placement could be on stands along the back wall or possibly you can experiment with placing them on the floor there and firing up toward the ceiling.
Any further changes, such as height or wide speakers, can be left for later.
