Also, thank you for the thoughts. I can't believe I'm doing nothing wrong, but thank you.
Victor's tests came back yesterday. The vet is 70-80% sure it's FIP. If Victor were an only cat, she'd probably send him home with antibiotics and an appetite stimulant. But we don't want to expose the cats any more than they already have been.
There is one bright spot. His lungs are clearing up. So apparently there was pneumonia on top of whatever else is going on. I think he's walking a little more steadily, which may be rehydration or may be improvement in his neurological system. At any rate, he enjoyed our visit last night. I suspect at least in part because we brought one of his favorite moist foods (Friskies Elegant Medley with greens).
We're going to try exploratory surgery tomorrow. If it isn't FIP, we hope to figure out what's going on. The downside is, if it's FIP, the surgery will cause him to deteriorate more quickly. But realistically, that means we put him to sleep this weekend instead of next weekend.