(Because some asked....)
Laev's bloodwork came back about perfect. While this is great from a health standpoint, it's frustrating because it doesn't help us identify what the mystery ailment that's sapping energy might really be. Her thyroid, enzymes, cholesterol, everything is just textbook normal.
The only thing that's off is a very slightly elevated white blood cell count. We don't think it's a low-grade pyo, or UTI, or other thing; it might be, if we had to guess, a parasite infection or an allergic response. As she's showing no other allergy symptoms, I voted to order a "parasite purge" on the off chance she got an icky case of worms somehow.
In the meantime, B12 seems to help her with her energy and general outlook, and the vet relieved my concern that it would mask further symptoms. So I'll try to maintain that as needed and of course I'll keep watching for anything more that could help us help her.
Thanks for your concern!