Well, Molly's blood count has gone back up since she spent some more time at the vets' place. Still no word on a cause, yet. For now, about all we can do is hope the current round of medication'll work. Hopefully, we won't have to make the hard decision.
We're worried this might be a genetic thing, rather than an environmentally-caused problem.
SUPER UPDATE: The vets have determined it's a blood parasite. Molly's doing pretty well so far on antibiotics. She's more alert, now. Now we just need to give her some recovery time followed by lots of rounds of chasing a point of laser light on the ground. All the iron and resulting appetite increase made her fat.