1. Should have plenty of power in most situations; very little difference from the 3805.
2. Sure; two front speakers.
3. Yes; run them in parallel(if impedance permits)using A+B on the main speaker channels, or assign the 6th and 7th back surround channel amps(if not being used for back surround speakers)to power the mains in another room.
4. You should make use of all your speakers, even with 2-channel source material. 2 channel material contains(in varying degrees)ambience which was recorded coming from the sides and back, not front. Processing such as DPLII and Logic 7 does a good job of extracting this ambience from the front and sending it to the surrounds where it belongs.
