I'd say mount the screen on the slopped ceiling and just tilt your seats upwards.
Wow, you really have a tuff install, the screen is really going to be very low. Normally, when seated in the front row, your eyes should be 1/6th to 1/3rd up from the bottom of the screen. In your scenario you will be looking downward, very uncomfortable, and you won't be able to recline if you get reclining seats.
The back row will have to have a very very high riser. Mine is 10" off the floor to accomodate a screen that is about 40" off the floor, so the heads of the people in the front row won't interfere with the back people.
In regards to the center, yes it can be angled upward or downward to point to your ears. Normally, you want the center tweeters in line with the left/right speakers, but this is not always possible. I would vote for hanging it from the rafters and pointing downward, maybe you can also mount your bookshelfs as well on the ceiling pointing downward?
So you have no other walls in that room for the screen? I don't think you will like the screen that low, especially for the back row seats.
