How are the current in-walls positioned? Got a pic?
I wouldn't bother with the ML sub, you can do better for $700. Look at Hsu Research, Rythmik, SVS...the sub doesn't need to be the same brand as the speakers. Even then, for a room of that volume, a single small sub may not be sufficient, and I'd include 12" subs in your space as small. Maybe spend less on the avr and move that money to subwoofage. Try shopping avrs at accessories4less.com.
With a wireless surround (which are to the sides in 5.1, rear surrounds come into play with 7.1) you're still going to have wires to the speakers and power cord to the remote amp. I'd never buy Bose anything. Maybe just start 3.1 and see if that suffices....