In response to your 3rd "or", you can definitely get your sub to work in 2 channel mode...most do, so long as you're using bass management with your receiver. The only way it wouldn't work is if a) you have the sub set to "no" of "off" in your receiver settings, or b) you have it set to "pure direct" or "source direct"...I can't remember which Denon uses. My Marantz has "source direct" so I'm pretty sure it's called "Pure Direct" on Denons...basically, that just bypasses all bass management in the receiver and sends full range to the speakers.
So again, yes, you can get the sub to work in 2 channel mode. For your fist two questions, I let someone else attack those as I've never used 2 subs and being as such, I've never experimented with daisy chaining them.
Good luck.