My vote 7.1.2
The sloped ceiling with a sharp apex at the rear of your room is going to cause problems if you place ceiling speakers nearby. You are going to get a pronounced "cupped hands" effect on your response in those channels with an after ring. The energy arriving at the listening positions will be almost all reflected energy off the rear wall. Not ideal for a discrete object mix.
Nick's 7.1.2 diagram layout is near ideal. The location of wall mounted M3s for rear surrounds will cause hot spotting near listeners. I recommend widening their positions slightly and aiming them at the main listening position.
Sounds like you are accounting for your ceiling slope when positioning your ceiling height channels.
The 2nd order bass width null will be shifted towards your sliding door wall by a couple of feet. You will have a hole at 39hz in the mlp if you put your subs on the left wall with wildly varying response over the listening area. They should go under each window on the front wall with the left sub output lowered to compensate for boundary gain.
So, predictions for rear height channels: I stole this image from online (sound and vision) and edited it to illustrate what I think the trend on the response will be. This is the best case scenario for ceiling mounted rear height speakers. Likely you can add pitched ring too.
This is all based off some quick napkin math and acoustical theory. In the spaces I have been involved with the math usually follows trends in real (rectangular) rooms though.
What do the "openings" in your room open to? This will affect your room decay times and low frequency behavior to a huge degree.
I hope you have a good designer. The rear ceiling corner is an issue. (The issues I brought up are obvious and an HT designer should spot these right off.)
