I agree with John about allowing at least 3", I would go more, to let the low bass filter into the room. Not sure I like your idea of having them back up to mineral wool, this may absorb to much of what you DO want to filter into the room. I have Owens Corning 703 cut into superchunks from floor to ceiling. My Axiom m60's are placed between them and the screen, but not in front of the bass traps. I also am not sure why you want to cover up your mains? Depending on the fabric you use, it WILL possibly affect your various low/high/mid frequencies.
