If possible, it would be nice to leave a little bit more room between the rear couch and rear speaker. Are you going to build a platform for the rear seating? I also assume your going to put a projector back near the back wall? It might be nice to move the couches up a few feet, and have a walk way behind the back couch so you can reach the projector. You could put a lamp in the corner.
The drawing is not to scale I assume, I'm worried your screen will be about 8ft wide, which might be to small for 18ft back seating.
I would not look at in-wall speakers, for surround, stick with the Q series 8's if you can afford them. Purchase from the factory outlet to get the 10% savings. Not being on a wall will not cause any problems, 3 of my Qs8's have no wall behind them.
