I can see that getting a plan might be the hardest part of this entire process.
If I set my seating in 1/3rd, 64", (or back 128") and my right side wall disappears at 94" into the room that leaves approximately 33" in front of me with no right side wall. Whereas you only have 18" in front of you with no side wall.
On top of that my entire room would be off centered which I'm kind of a freak about. I'm weird like that. All my light switch covers in my home have the screw slots facing the same direction- horizontally. I think they call that OCD. I was going to move all my canned lights over because right now they are centered to the room being 15 feet and would have been off center to the room as I had planned to set it up. Anyways....
Couple of ideas come to mind:
1) What if I flipped the entire room to face the East wall? There are three windows on that wall, and the foundation (same dimensions - 45"hx8.75"deep) runs the length of that wall. Measurements are taken from ground so 45" up I have an extra 8.75". This is not an issue for layout unless it's an issue for sound. See picture.
I would insulate/block window light with foam insulation panels. I would then frame it all off from floor to ceiling using 1x4 s, and treat with insulation above the top foundation line (45" up to ceiling). Using 1x4 would allow a very little intrusion to measurements. The 4" would lie flat against wall. I could do this on south wall as well. I'd have a black velvet theater. Paint the ceiling to match.
2) Really don't want to, but I could, throw up a wall to make my original "imaginary" 12x16 room a real 12x16 room. Door at back of room.
Option 1 would still put seating at 10' from screen, leave 5' behind for rear surround. If I had a 12' wide four seat setup I'd have 2' from edge of seating (not center) on each side for side surrounds. I'd still have the L shape but it would be behind me now.
What are your thoughts? I really appreciate all your input as it is raising issues I would most likely not have caught.