Add egg and/or milk to her diet. Also, if she is around other male creatures, she may get upset. My hamster, Lily, ate a few of her babies because there was a male dwarf hamster present in the room. It's strange, though, that an animal as social as a gerbil would kill their 4 week-old...It's terrible.
As for keeping a baby with no back legs...Keep a watch on the wounds. Make sure nothing's bleeding. Perhaps get a good anti-biotic. That's about all I can think of. Perhaps address the problem to your vet, however, I would not recommend going in with the baby, as the vet's first notion would be to put the baby down. Give it a chance, however, do not expect anything.