My TV viewing area is half of a long room which is about 20' wide.
Due to one side being all glass, I'm forced to put rear-speakers in
the 10' high ceiling. I have 8" JBL in-ceiling speakers with
swivelling tweeters.

Where should I mount them? Specifically,

1. Should they be horizontally in line with the couch, or somewhat
behind the couch?

2. How close to the sidewalls? (as I said the room is 20' wide)

3. Where to point the tweeters? Directly at the center of the couch?
Straight-down? Towards the side-walls?

Any advice is most welcome - I don't want to make too many 10" holes
in my ceiling!!!!