For your requirements, I'd:
1. Ignore the cross bike suggestions, other than they're the bike du jour on STW. I can't see how one would ride as well as a dedicated road bike.
I ts replacing an HT and a road bike ... A CX bike will manage most general off road stuff fine that the HT did , plus with a set of slicks on a 2nd pair of wheels ( as mentioned by OP ) will hoon along the road all but as well as any other road training bike.
A CX frame is likely to fulfill the options for all the requirements he lists in terms of what it would need.
Mud guard clearance - plenty and cx frames generally have mounts
racks - if you've got mudguards, you can fit racks ( generally )
tyre clearance - plenty
He also mentions lots of steep hills, a CX bike would likely have slightly lower gearing than a road bike, making it easier up those hills as for towing.
Need a bit of help- why, apart from tyre clearance, might a CX bike be the way forward. I know nothing about them geometry/handling wise- above?
any thoughts on how a typical CX bike would compare, ride-wise with, say, the Equilibrium or Pearson Audax suggested
The typically have a slightly longer wheel base than an all out racing road bike plus maybe a slightly slacker head angle ... makes the general ride a little less twitchy steering wise and a little more supple.
The bars may be slightly higher than your typical road bike too, making the position a little more upright, easier on the back.
But then again, those a features that you could also find onan audax bike too.
Personally, I poo poo-ed the idea of CX bikes for a long time, however, I got one on a bit of a whim, and its turned out to be a great bike. OK, its not quite as fast as my all out road bike, but its considerably comfier, smoother and the ever so slight slower pace is more than made up for by the nice smooth ride. I'm now ditching the HT frame in the garage for it.
You mentioned its replaceing an hardtail mtb. Are you expecting to go offroad at times with the new bike? would you be expecting to be able to go over some rough stuff? If so, then the CX bike.
If you're not expecting to go off road atall, then a tourer/audax may be the better option.
