I am curious why you did not choose to go length wise with the screen on the end wall. I can only see so much with the photo's you have. Basements and the accompanying venting can be a real PITA . I just bought a house and had to go from 7.1 to 5.1 and honestly I don't miss the extra speakers but having them in the closet bugs me LOL. I have QS4's in the bedroom and QS8's in my movie space and they work great as side surrounds , I have found that mounting them up high near the ceiling works best. I would say some M3 or M22 in walls at the back would be your best move.
