She is going back and forth to corner to corner. I know it must be difficult on her. There are two other moms in there, with their own litter. So, hoping for the best. Yeah, she had a placenta in one corner, I thought it was just a bloody and dead baby, but no.
If you can't get the mom to pee on the adopted baby, is covering it in their bedding another possibility? (I like seeing these threads with other rodents helping each other out).