I am not a fan of inceiling speakers but you really look like you have no place for wall mounts, other than to ceiling mount the QS series, which works VERY well, you might possibly be able to mount them above the windows and on the stair wall?
If you must have in-ceilings I would suggest moving the rear surrounds a little further back and try to find in ceiling speakers with adjustable tweeters, this will halp with the surround feel.
I am more or less looking at the same set up as you across the front and would really like to use in cabinet M60 or 80s built into some bookcases on either side of the TV mount and then add an in-cabnet VP180 mounted above the TV angled down towards the listening position. I plan on making the tray ceiling at the correct angle for the VP180 that way it can be part of the tray. I am going to mount QS8s for my surrounds as I have walls I can use.
