I think for your side surrounds if you were to mount them about mid way between the back wall and beside the couch you would achieve the best surround field, it should be possible as you were thinking of ceiling mounting them anyway.
So far s the LF main, you will just have to play with it to find out how much it affects it. I have my M22s upstairs in such a positon and I haven't noticed much of an issue, then again I don't listen critically up there either.
For the sub, the back corner is probably the better place as the forward/mid wall position kind of duplicates the LF speaker situation, with the RF main sitting in behind the sub. I also found that having my sub shooting across the front of my mains didn't quite sound right in my room.
