"sensible" roads: the Audax routes are planned to take nice minimal-traffic routes, in general. Obviously in England, particularly SE England, you're going to encounter cars etc, & rides do go through villages & occasionally small towns (especially for controls/cafes), but by & large it's all little roads & countryside & so on. If "sensible" means "avoiding of upwards gradients" then not so much ;-) (I do not in general do Very Hilly rides, but most Audaxes have a degree of Scenery going on!)

Bents are *entirely* acceptable. As is basically any other form of human-propelled cycle-ish thing (I've seen folk on fixed, MTBs, racing bikes, tourers, hybrids, recumbents, trikes, etc etc. And some variations in the middle.).

The doing-a-loop is a little irritating for those of us who get to places by public transport, but it does mean that people who get there by car (most folk, really; and tbh they're easier to do by car because tend to start in Countryside and/or at unsociable hours) can do so, and also that for some rides you can leave kit at the start/end & pick it up later. Dry/clean clothes, that sort of thing. Although I rarely bother. Easier for the organisers, too, I guess (they'll likely be close to home so when they've finished checking people back in they haven't got far to go! In the case of this ride the finish *was* someone's house...)

Weather is kind of unavoidable ;-) although you can always not-start after entering if it's too vile.