1. I would run cat6. Rated for higher speeds.
2. Wire every room for sound and run it back to a central location. Easier to control that way.
3. Id worry about wiring for 7.2 then think about the system itself.
4. Dunno but I bet the electrician could do it.
Also run conduit or a pipe from the attick all the way to your central drop room which sounds like your new office. That way you can easily add things if you'd like. Also one from the basement if you have one.