So Mittens is home and diagnosed as hyperthyroid, apparently a common problem for older cats and one treatable by daily doses of medicine (which we will be starting tomorrow). Luckily Emy just went through this with Zoe and has been a great resource (as have the cat people at work). I kept thinking of that saying about "Outside of a dog, a book is a man's best friend: and inside a dog, it's too dark to read" as I got an extensive tour of the insides of my cat via a series of x-rays. But other than the hyperthyroid and some unexplained (but not too worrysome) abnormalities (which we'll keep an eye on but not worry about yet), she's in really good shape and well adored by the vet staff.
She and Bonnie/Mitzy have gone back to hissing at each other, as can be expected after a separation and the bringing home of new vet smells, but hopefully they'll get back to the progress they had made soon.
There's not much info on their page, but this is the place where I got Zoe's meds:
http://www.vetrxrx.com/
Veterinary Pharmacies of America
I can't find the paper that came with it, but it was pretty cheap. They ship it via FedEx and require a signature, so you'll want to get them to deliver it to work or something.