Good news. our kitten hadn’t perked up by Friday, and had gotten noticeably skinnier, so we got her an appointment with a vet. It turns out she had been going through an upper respiratory infection with a side order of goo in her left eye. She got three prescriptions: one for the eye, one for the infection, and one for the diarrhea that I think I forgot to mention earlier but was awful. all have been extremely helpful and she is now doing kitten-type things again.
The veterinarian had mentioned that we might want to have her tested for feline leukemia, since she had lost weight and was down to 1.7 pounds. Seems like that won’t be necessary though. We got her some high calorie kitten food and she has been inhaling that as well as eating the other cat’s crunchies. We’ve given her the name, Clara (because my wife vetoed Baconator).
Thank you to everyone who gave input. This community continues to be a reliable resource in so many ways.