Blood test was normal (good sign).
Hauled her in again for a urinalysis. That was normal too.
But she was still off her chow, unless I fed her baby or people chicken.
Got some calories in a tube from the vet and started hand-feeding it to her.
I took her in for a followup yesterday. Vet detected a very slight heart murmur and we decided to go ahead with the ultrasound (next diagnostic step).
I now have a kitty with a shaved belly who has a slightly thickened heart muscle and a slightly thicked gut lining. Neither of which the vets think is significant enough to treat at this point (meds). No tumors or blockages of any kind. ("No extra bits" as I said.) Who has gained almost half a pound since the 8th on the Special Treats regime. And cried for food the minute she was out of the carrier, and scarfed it all down.
So it's back to being watchful, but I am relieved.
It's been a seige of taking her in ... next regular visit (providing she doesn't start exhibiting symptoms beyond being sluggish and eating less) will have the work, including an ultrasound, scheduled on the same day.