Hey NikNox -- if I can gently advise something, try to let go of this expectation/desire that if she would just scream out her issues directly to her mother it would somehow relieve things for everyone. You'll just make yourself nutty with it.
For one thing, she won't be doing that any time soon, no matter how many times you tell her she should. She has clearly not truly accepted her mother for what she is, what she isn't, and what she never will be. She is still trying to win her love and attention because that is one hole that can never be filled by anyone or anything else, and for many of us, that hole is the basis of our own self-worth. Six years of a functional environment is not enough to counter twelve years of having it drilled home in every moment that she is not good enough to be loved and cared for by her own mother the way a mother is supposed to do.
Now, that doesn't negate the good you've done or the difference you have made -- I'm merely saying that one time of having it out with Mum directly isn't going to fix that. It will take a lifetime of learning to accept and understand that she deserves and is worthy of love DESPITE the fact that the one person who is supposed to do that unconditionally, didn't. It starts with acceptance, and she isn't ready or mature enough to do that. So she is going to keep trying to emulate Mum, be "friends" with Mum, and get her validation that way. Meanwhile, she is going to keep lashing out and using you because you are perceived as a safe place to release all that hurt and anger.
That doesn't mean that you have to live with it, of course, and I think the sooner you and your husband can agree to release her into her own care despite the potential consequences, the sooner she will decide that the pain of trying the same thing over and over again with no result is less than the pain of trying something new. Right now you're all stuck in a cycle. If you want peace, someone has to break that cycle, and it isn't going to be her.
I know this is very frustrating for you, and I'm sorry. Having grown up with an alcoholic mother and a severely codependent father, I am not making excuses for her behavior, just warning you that this is not a knot that will be untangled easily or quickly. Facing the truth of a mother's limitations, and really learning to believe that it wasn't your fault that she couldn't love you and care for you the way you needed, is a lifelong journey.