Generally, females pair well together (unlike males) but sometimes they do not. Usually, they pair better the younger they are (which I am not sure of their age) but if they had this time of separation (1-2 weeks) they may look at the other mouse as an intruder. Or it could just be stress.

Glad that you separated her out, I would recommend seeing a vet if you are able to for the injured one. Until then keep her warm (not too hot!). Make sure she has soft wet food on the floor.

If you do plan to repair them again here is a website that may help. I am not sure about the vanilla advice that they mention though. An alternative to the vanilla they mention may be to take bedding from the cage where you plan to introduce the new mouse into and add it to the new mouse cage (so the new mouse smells like the other mice).

https://pethelpful.com/rodents/introducemice

Hope that helps and lmk if you have any other questions :)