Wow Venkat,

Lots of questions.
Your room is almost exactly the same size as the HT I'm building (See HT in-progress). Since it's not finished I can not speak to how everything will sound when the room is completely tuned. Since I'm interested in having both a great 2 channel experience and a great HT experience my main speakers are not behind the screen but on the screen edge (about 2.5 feet from the side wall and something around 3' in front of screen). The center will be behind the screen. Probably a VP180 on-wall if I use masking. If I don't an M80 on-wall.

I've tried out a system in the room (untreated) to get a feel for how I wanted to set it up. For this test the mains were LFR actives, the center a VP180, side and back surrounds M5's and the tops M3 on-walls. Sounded great.

I would not worry about the VP180. It sounded great from every seat and it's a good size for your screen. But I personally think having 3 matched speakers for your L/C/R is the way to go if they are behind the screen. Take your pick of on-walls: M5, M60,M80. The reason to go with a horizontal center is only because most don't have an AT screen and have to put the speaker below it.

Every on-wall I've heard sounded fantastic. The in-walls I'd personally avoid. I've tried a pair and they are a huge compromise. Consider putting a false wall 6+" off the front to hold the screen. You might want to make it of AT fabric so you have the option of placing the speakers and acoustic treatment where ever you choose.

Lastly, I understand that the optimum screen height is with eye height somewhere in the lower third of the screen. My thinking is if your natural seating position angles your head quite a bit it's the lower 1/3 of looking straight ahead.

Axiom sales vary greatly ... sometimes ok and sometimes it's just too good to pass up. My guess is that they will do something for xmas.