Sonus faber choices - help me decide

-Well both will be much better than the B&W CDM 9NT so no worries there but if you have the means always buy the better speaker.  Futuras Is an endgame speaker IMO  

-the first question I have is how do you use your subs? Do you highpass (cut the lows to the mains) if not you should be. This will play a big part is what way to go. The main difference between a lot of speakers is just the bass and if you are cutting it off anyway it matters a lot less.
 
-the soundstage and inner detail will most likely be stripped out a bit by the denon it will just sound like it has less scale. I also have a combination two channel home theater and speak from experience (I have had many systems with denon, marantz, b&w 703s (newer version of your speakers) and I run duel subs (picture of my system in my profile). Anyway the kicker is you probably use your denon for bass management and that will make it hard to go to a two channel pre-amp if you ever want to (you should) because you would not only need to bypass the surround through the two channel pre amp but also through an external crossover. For what is is worth I do just that. Have a two channel preamp/amp with “external” crossovers in the subs and bypass through the two channel system for home theater. Works great, best of both worlds but is a lot of wires and equipment. Also family members will never be able to turn it on again, you can decide if that is good or bad lol. 
 

Well I would take the better speaker every time. 
 

yes you can connect most subs to both a two channel preamp and receiver at the same time. It is important to have the inputs sum. If they don’t it will not work so read your manual or give them a call. I do this with my JLs and it works well.
 

I would also encourage a higher crossover. Your subs are better than the speakers below 80hz for sure. Try 60, I bet you get more impact and lose nothing. 

Good to hear and good choice. I love the nova III and the V will be great.

The amp is fine honestly to get you going and I would start by adding a two channel preamp. One with a home theater bypass would be simplest. If it does not have one you can just set the volume the same every time (12:00,3:00 etc) and use one of the inputs to pass through.

I have had several Rotel amps and still use one on my surrounds. I have done direct AB comparisons against Bench Mark ABH2 (I no longer own) and my current McIntosh MC462 and the Rotel is fine. It has plenty of power. They are a touch bright (no brighter than say Bryston) and a little cold/edgy in the upper mids in absolute terms but really start with the preamp or just get a good DAC if you just use digital music those surround processors suck the life out of music.

When I demoed the Nova III it was on the new Classe’ delta amp and matching preamp ($$$$) but it was pretty great.

Yes use the bypass option to a preamp or DAC.  Pricy home theater processors are still just ok (not great) and get outdated real quick. Honestly I would try some budget ones first. Schitt or Benchmark might surprise you.