Hmm, so I deactivated the Apothecary satchels mods. That didn't work and other than the USP, it was the only one of the small number I am running that would've possibly interfered. (The others are strictly chargen mods - eyes, hair, eyebrows - no other armor, clothing, house or gameplay type mods.)
So I unchecked the Hearthfire esm, loaded up the game, ran off to Falk Firebeard and he had the living room available again, so I purchased it and now Proudspire is looking like it should with all the vanilla house stuff.
I'll try re-activating Hearthfire. but one other question - I am running the Unofficial patch off of the Steam workshop subscription - I cannot tell what version it is, and I thought I understood they updated automatically? If not, should I just delete and resubscribe? I haven't done much with the workshop or much with modding Skyrim anyway (obviously...)