Nancytracey, my heart goes out to you! I know you're worried about gd and must miss her very much! And I feel for gd also! First, her mom adds a new member to their family unit, which she doesn't like - and then she's co from family members that she knows and loves! I understand that your dd wants/needs a new man in her life, but I don't get how she could tear her (troubled) little girl away from family that mean something to her.
I think it's jumping the gun to say that the new man is controlling or coercive though. It sounds as if, with the best of intentions, Nancy, you interfered in their parenting - or that's how they see it - so they were both upset. It also sounds as if they responded by limiting contact, and you, understandably, made an issue about that. I say, "understandably" because you were, clearly used to more frequent contact. The changes must have seemed drastic to you.
Unfortunately, it is probably because you balked at the lowered contact that dd cut you out completely. I'm so sorry it went down that way, and hope she softens after a while.
I'm not clear on how much contact dd has with her siblings. I take it they text, but she has blocked them in all other ways? It's possible that's because they keep defending you and trying to get her to change her stance. I know it's hard, but Imo, you all need to back off a while.
I'm glad gd is in counseling. Please trust the school to help her. As for mediation, I doubt dd would go. Idk, but Imo, for mediation to work both sides have to have some leverage. You have more access to gd to gain from mediation, but what does dd have to gain? Do you see what I mean? She has authority over who gd sees and doesn't see. Why would she agree to negotiate that?
So I agree with those who say to try stepping back and just waiting for dd to (hopefully) reach out to you. It may take her longer than you would like, but chances are, it will happen.... Patience... and hugs!