We took Pete to the vet Tuesday, and they ran a bunch of tests on him. But the tests revealed nothing - no infections, no parasites, no interesting levels of anything in any of his fluids or excretions.
The vet did feed him some special prescription-only (!) cat food while they had him, and he tolerated that pretty well. So she gave us some to take home and it seems to have helped quite a lot. He's still not totally right, but the food does seem to stick with him better. So we're giving him the special food, and antibiotics, and probiotics, and Pedialyte (for hydration), and canned pumpkin (for fiber). He's no longer dehydrated and might even be putting on weight. He is always absolutely ravenous and we feed him every few hours. Still, even though food doesn't rush out of him, it comes out ... softer than ideal.
So I'm thinking ... food allergy that hasn't quite settled down? IBS, IBD?
Baby [cat] steps, I guess.
He still officially lives in the bathroom. But he's sneaked past us a couple of times and has seen most of the house and has bumped noses with Pepper. Pepper seems cool about the whole thing. Thankfully. She isn't hissing and spitting like Tricksey was when Pepper showed up.
T wanted Pepper dead, the second she first saw her through this crack in the door.