The hospital tank has been up and running for over 24 hours now, and the severely injured black molly fry is isolated in it. The malachite green treatment appears to have been successful; there is no sign of fungus anymore.
The bad news is that she dropped a significant portion of her tail, probably because everything important had been severed. The good news is that she is eating and making attempts at swimming, compensating her damaged pectoral fin with her tail bent to the side of her good fin. (The physics of one fin and the tail bend cancel both cause the fish to turn, but they cancel each other out.)
Her health appears to be stable. While I wish she was never attacked, I feel good about her chances of survival. She’ll never have a normal life for a fish, but at least she’ll still have a life.
