This morning we took her to the vet, and her numbers are much improved. He did an ultrasound (so now she's got a shaved belly, oh the indignity!) and she's got chronic kidney disease, but still has 25-50% of her kidneys. So hooray! It will shorten her lifespan, but it can be managed, and she should still be with use for a while yet. (And it's probably genetic.)
So now both our kitties get special food, and every day she gets a pill (which we can dissolve in a little water and add to her wet food, because ain't no way she's gonna take it otherwise) and some phosphorus-binding liquid (also added to the wet food), but otherwise things are mostly back to normal.
*Whew*! It was an emotionally exhausting and expensive ordeal that felt like it lasted for about three months, and I'm glad to have it over with.