1. DVI and HDMI are interchangable with adapters, though DVI is video only and HDMI carries both sound AND video. So with an adapter, or a cable with a built in adapter, you can use your reciever to do the switching. I'm not a TV person, so I can't comment on your choice as being good or bad.
2. I would say 12ga. Yes it's overkill, but then you won't ever have the, "Should I've gone with bigger stuff?" thought coming back to you. Plus, you can buy a boatload one kind if wire and use it all over the house, even for really long runs to the other end of your place (if you so choose).
3. You can make your own, or you might check out the options at
Parts Express. They have a few in the lower budget area. Most everywhere else I've seen starts around $100.
4. No, it shouldn't be. Just keep in mind that it's going to take quite a bit of experimentation (unless you're really good with lucky guesses) to get placement, levels, and phase dialed in. Done correctly, two subs can yield awesome performance.