So it turns out the reason I get stale air and that the room doesn't heat well is the return isn't really doing anything. They will be coming out to make a repair.
Luckily they can just run an elbow from the return duct in the unfinished space to the top of the wall in that cavity. They'll cut a hole in the back of that wall -- coming from the other room (unfinished space) to complete the connection to the return.
They are also going to change out the 3 inch flex ducts to 6 inch flex ducts to give me more supply in the vents in the theater room.
Hopefully once I get adequate supply and an actual working return, the room will heat better.