Nona became sick on June 7th, the main symptom being that she stopped using the litter box; though I never caught her at it, she was behaving oddly and tests showed she was sick, so it seemed pretty likely. She took a course of meds that should have stopped the problem, according to the vet, but didn't.
It's controllable, sort of, by putting a rug or towel target for non-litterbox use in the upstairs bathroom (to deflect from the bedroom carpet, which hasn't been hit but is not too dissimilar from the rug that was soiled, or the bed which HAS been soiled, while I was sleeping in it - yuck yuck yuck!), but I don't want to reinforce that as an alternate or continue to wash pee out of things, so have continued trying to put an end to it - and meanwhile, kept the bedroom door closed all the time to prevent incidents.
The next step of the plan was to return to the vet, but first I needed to do an isolation test to try to determine for certain which cat is responsible for the continued problem - Chirpy never was tested, and it was possible that she'd also become sick and Nona's sickness had masked it, or even that once the behaviour started, she took it and ran with it (I didn't think these things were likely, mind, but I needed to rule them out).
I finally did this last night - put a fresh target in the bathroom, then set up food, water, a second target and a litterbox in the bedroom and closed Nona in with me. On a suggestion from loh, I used a brand new box that I purchased last night, along with the brand new litter. Then I went to bed, though I had some trouble sleeping, because I was afraid of a repeat pee awakening.
The test failed in the best possible way - NEITHER target got used. Nona definatively used the litter box in the bedroom. She also had no problem being shut in all night with me, which is weird, though whenever I left the room she yowled as expected.
I let her out this morning, leaving the new box set up in the open bedroom, to see what happens while I am at work. Tonight, I'm going to replace one of the not-new boxes and scrub the heck out of the other one and, again, see what happens; I had only changed the litter before, so this really could be the solution. I don't want to get too invested in hoping the problem is solved, but it might be.
Other people's cats have been far sicker than this, recently, and I am grateful that I'm dealing with something relatively simple - but this whole thing has been exhausting and kinda disgusting, and I would be super grateful if it were over.