That's where the peaks and nulls are worst. I understand the desire to get back from the speakers a bit, but a wall right behind you really should be treated with absorption or possibly diffusion.
1. Symmetry in the rear of the room is less important than in the front. But a door doesn't have to preclude a bass trap. The photo below shows one solution - traps on the door and wall so they're at least near the corner.
3. No need to seal, but an air gap the same as the panel thickness helps a lot. As shown in the photo.
--Ethan
