Another welcome to the forum. I came seeking advice, but stick around for occasional doses of my own desire for therapy and counseling
Yes, your room is small compared to many builds, but you have more than enough room to construct a dedicated theater space that performs well and looks great. That said, I'm biased. My room is similar in size to yours. The room was 16'8" x 12'2" with 8'9" ceilings.
This is what the room looked like after spring rains led to a basement flood. This is a view towards the future screen wall:
After quite a bit of planning, professional design advice, expert advice from BIGmouthInDC, great input from several forum members, and a lot of sweat equity, the room looks like this:
You've done quite a bit of thinking on layout, but before you start swinging the hammer, a couple items to reduce significant rework and/or "I wish I had done that differently"
1) Figure out your lighting plan (e.g., screen wash, primary room lighting, accent lighting)
2) Determine if you need any HVAC modifications, both supply and return, to keep the room comfortable
, after you dramatically shrink the size of the soffit around that duct, you could incorporate it into your ceiling design. Perhaps create a double tray ceiling, one tray between the screen and the HVAC with another between the HVAC and the projector. Then, you could put recessed lighting in the side and rear soffits. Put screen wash lighting in the front soffit. And consider accent lighting in the 2 trays using LED strips.
Side note: example of minimal framing around an i-beam to reduce ceiling height loss. You might be able to do something similar with your HVAC duct:
, the HVAC duct should have more than enough supply air, but you may want to increase the register size to minimize the air noise. Is there a return in the room? If you're not "soundproofing", you could use the space under the door as a return. If you decide to go the sound containment route, there are a few other considerations for both the supply and the return.
With those two plans decided, you may decide to do a bit more demo so you can pull wires and modify HVAC before you switch to building.
Good luck, I'll be following