Ah. 0 Fahrenheit. That's quite cold - too cold for an 851.

 The Fugu I would guess is far too warm for the temps you're talking about. I have the 851 and it's good (I mean really good) down to -5.

 +1 for Boras if you're going for tubs. For clinchers, Edge 45 rims would be top of my list, followed by Reynolds. Corima make a deep carbon clincher rim, IIRC.

 I'm 82 kg; I've been riding it for over a year on some pretty bad roads in London and SE England; plenty of bumps, potholes and cracks in the road.  It's been fine. Never felt even close to having an issue with it. 

 Stem flip. That way you don't have to worry about repositioning the bung in the steerer.

Spin bikes and trainers will all drive you mad with boredom sooner or later. Rollers?

404 clinchers had a few bad reviews a few years back - rims cracking, braking surface delaminating. Dunno if they're better now, but they are a heavy wheel; too heavy for an R3SL. FFWDs are heavy too. Wouldn't bother myself. I'd look at Campy Neutrons for clinchers, or Hyperons for carbon clinchers if you want a factory wheel ...

If you look for Cervelo's unique selling points, they are aero bikes and light bikes. I can't see aero qualities holding much attraction in the cross market, and lightness is more likely (IMO) to be viewed with a degree of suscipion by crossers - we get enough questions here from roadies about whether an R series will take their weight.

If your rear brake is rubbing, opening a Campy caliper as you ride is s lot easier than opening a Shimano caliper. I've managed to rub my fingers into a moving wheel a few times doing a Shimano rear on the fly.

I'd say 303s offer an advantage going uphill, but 404s have the lead the rest of the time. So unless you're planning a lot of climbing and not much descending, 404s have it. That said, I'd go for Boras, or Corima, Reynolds or Edge rims built around my choice of hub before thinking about Zipps.