1. Yes but it will be a pain in the ass. You have to plug in monitor, overclock, start mining. Unplug monitor switch cards, repeat. And every time a miner crashes or you restart you have to do it again.
2. Yes you can, there are some problems with crossfire that I don't recall offhand, but as I remember it lowers your hashrate and can be buggy. Someone else will have to give you more details though.
3. See above
4. Crossfire does not change power consumption.
6. No it does not. All windows versions should mine at roughly the same pace, if you disable aero and all the graphical desktop crap and don't watch a lot of movies and things.