Your middle choice of the back wall should work just fine. Room correction such as Audyssey will compensate nicely for any anomalies & the QS8s are certainly powerful enough to get ample sound to your listening area.

My QS8 mountings are far more problematic than that. Mine are about 8' back & are off center behind my MLP due to the shape of my room. With Audyessy's corrections, they work well, although probably not ideal...

TAM