If it were me, I would move the QS8's to the sides and mount them on ceiling brackets since you have a window on one side. The surrounds are key to creating that enveloping feeling which you will negate a bit by having them behind.
Since you already have the rear wired just put some bookshelves up and you have a 7.1 system. I am assuming from your setup the cost of an extra set of small speakers is not an issue.
You have a great room with an awesome home theatre setup. Why go to all this planning and expense and then compromise on it in the final stages?
It sounds like you are hung up on what you have done already. If you just want it to be done you will still have a great sounding system, and depending upon how much you are into the theatre experience, you may not even notice it isn't quite right until you are in a properly setup room. I think most here would move the rears to the sides at least.
