Tom, I think you're right on about the cross check. Vintage retrogrouchery aside, that's a super versatile bike (and IMO good looking for a modern bike) and now that you rock the trailer your hauling capabilities are much expanded. Your idea of a fast wheelset and a utility wheelset (like some big fat 26" to raise the gearing slightly) sounds like a good way to go for maximum bang per buck.

if you want to do some MTB'ing just get some knobby tires for it.

That, a 3 speed and a Folder pretty much covers the bases.