Question -- would it defeat the purpose if I built the bass trap from the ledge and up (subwoofer and speakers will be on the floor)
It won't defeat the purpose, but the general rule for bass trapping in small rooms is the more, the better. Also, extend the corner bass traps as much into the room as you can. The further out they are the lower they will absorb. Bass traps are most effective at the half wave length away from the wall. So take the distance from the front of the trap to the wall and double it and that is the frequency it will be most efficient at absorbing. Go as thick as you can for the corner traps as well.
Since you are putting in a false wall, you can bass trap the ceiling/wall and floor/wall corners as well.
2) Put a few panels of Owens Corning or similar on the back wall (behind the false wall).
I have seen everything from 2x4' panels back behind the speakers to the complete wall covered. I'm not sure which is best or if it varies by room.
I would post this description and your questions on one of the two forums I linked to. The guy who answers the bulk of the posts is one of the tech guys from GIK Acoustics and is quite generous with his advice and suggestions.
3) The False wall will be covered with a thicker black fabric that is not see-through so you can't see the studs.
Question -- this material should not matter correct?
It does make a difference. As the material starts to get heavier it will reflect more and more of the higher frequencies. How much difference it will make in practice, or in your room, I don't know. The guy from GIK will give you a much better answer.
#4 I like Micah's suggestion. Front porting would work best if you build the speakers into the wall. Play with the placement of the speakers before you build them in to get the best sound-stage.
#5 I would put the center up just under the screen. It probably makes it easier to center the dialog vertically. When I first set up my system, I had both my centers (dual M2s) on the floor under my RP pointing strait out. The whole sound-stage was pulled down. Angling the speakers up would have improved things, but I bet its easier if the center is up off the ground.
You have a very solid plan. Now you just need to tweek the details.
