If sound isolation is not a concern then yes, you can stuff with insulation and cover the bottom with fabric. If your going the whole double drywall and green glue route then I would say no as it would totally defeat your sound proofing efforts.
As far as subs are concerned, a simple test is to place the sub at the seating position and then going around the room on hands & knees finding the position where the sub sounds/feels the best. Tuff to do in an unfinished room but you can probably get a fair analysis when everything is framed out and insulation is in place and before drywall goes up.
Having said that, I simply had one place where my sub would fit and that was dead center below the screen. With bass trapings at the corners of the screen wall it has worked out well.
At a minimum I would move the subs foward from the corners to the false wall, and use the corners for OC bass traps.