This is what you do to protect her self esteem. Have a slumber party for her at YOUR house next month and invite the girls except for the problem girl and the girl whose mother dissed your daughter. Then that mother will find out how it feels for her daughter to go through what your daughter is going through. It's the only way.
Regarding the problem girl, when she asks why she isn't invited or when she comes over and wants to play, YOU should sit her down and talk to her. You need to tell her specifically what she has done (no generalities) to upset you and your daughter. You need to tell her that she cannot act mean to people and then expect all of them to want to play with her. If no one is talking to her about her behavior, she will not figure out how to manage it.
Your daughter needs to continue to stand up for herself. I would not have blamed YOU on this thing with the mother, if it were me, Kay. I would have been honest with your daughter that the other girl's M. is not thinking about this right, and give her real reasons that she can understand. Sometimes we pay a price for standing up for what is right, and that happened to your daughter. It's all on that M. for punishing your daughter and the other girl who stood up to bad behavior - not on you, and not on your daughter.
I'd revisit that issue with her.
Yes, she has to continue to be around that girl, but I do think that continuing as you are both doing will make a difference in the long run. Working with the school on the problem behavior will help too. Most likely the girl has an IEP and they can easily work in a behavioral modification program as well.
Tell your daughter that in a month, she can have her own slumber party, and make it LOTS of fun. That will go a long way towards helping with this, on multiple levels.
Dawn