Mark, one thought is that if you're going to be sitting about 11' away, more spacing(say 10')between the mains would be desirable than the mounting on the vertical mantel columns would permit. Of course, if that's about the only practical option, it'll have to do.

Yes, since you're sitting almost up against the back wall, the QS8s should be mounted on the back wall, about 3' above ear level and separated as widely as possible, since they're side surround speakers.

Enjoy the music, not the equipment.