or requires crazy cornering skills lest he fall off without the aid of ice.
The cornering skills are what makes it so much fun. Realistically a trike is out of the question because it is 10 to 15% slower than an equivalent bicycle.
Indeed, though a trike with fast road tyres is likely to be faster than a bike with studs, which was my original point.
A fast recumbent trike should compare favourably to his DF bikes, given that he's avoiding hills, but the efficiency hit from not being adapted to riding a 'bent would cancel that out. If a recumbent rider were doing the challenge, I'd be surprised if they didn't choose to switch between two wheels and three as conditions dictate.
Understandable that's he's taking the risk in favour of more mileage. Main roads are usually fine.