I'm considering MPX's for the surrounds and 650's for the ceiling. They must be in-wall/ceiling speakers for surround and height.

To be clear, you recommend the surrounds be in the rear wall and not the side, even though I can't angle them towards the listener? If they are in the rear wall, they'd be about 6 feet outside the listening area. Side walls would be about 10 feet outside listening area, but pushed into the corner.

To be clear, you recommend the surrounds be in the rear wall and not the side, even though I can't angle them towards the listener?

Yes. You said the side placement would not be behind the listener, so that's out.