Your post has given me pause Victorian, and I have a couple of thoughts about it. First, please accept my sincere condolences regarding your loss of your mother and the lack of closure that you have suffered. I can't even imagine how you must feel.
I mentioned having helped with some of my nieces and nephews. In those cases, the children were definitely being harmed by their mother (emotionally), and it was their choice to take refuge in my home, so to speak. I was aware that the cooperation that I received from them was due in part to the fact that I was NOT their mother. I just offer the possibility that this may have also been true for your aunt. Under no circumstances would I ever, EVER EVER EVER, send my child to live with someone else in the manner that you've described. I can imagine that this caused you to feel very bad about yourself and I am very sincere when I tell you that I know what it is to feel fundamentally flawed because if your own mother doesn't want you, who does?
The OP of this thread really gives me the impression of a very respectful and loving woman. The hostility that she feels is indeed something that her daughter can feel as well, but you feel what you feel. The OP has these emotional responses for good reason. Feelings can't be wrong, IMO. Only actions and behaviors can be wrong. Feelings, at least for me, are something over which I have far less control than behaviors or words. It seems strange to me that the eradication of the rude behaviors should not be a focus, but the very natural hostility and resentment of the mother should be considered to be the problem. At the same time, I can't stress enough that it is the BEHAVIOR that should be the focus, not the 'goodness' or 'badness' of the child.
I do very much agree that the goal should be to get the child through this time with their feelings of self-worth intact. Self-worth, to a very great degree, stems from being authentically appreciated for yourself. Ignoring or tolerating rude, obnoxious behavior does nothing to build the self-esteem of the child, IMO. Not to mention that many children feel out of control, and although they will NEVER admit it, they want the security of knowing that you are there to draw limits for them when they are unable to do so themselves.
I do very much agree that the transition from childhood to adulthood should be transitional, rather than, "You're sixteen, here are the keys to your new car", or "You're on your own now that you are eighteen." But I think much of that transition should be earned. A proper adult handles his or her responsibilities, owns the effects of his or her communications, cleans up his or her own messes and pays his or her own way. While I think that children should be spoken to in a respectful way devoid of belittling or ridicule, I do not think that the privileges of adulthood should be the result of hitting some arbitrary marker such as age alone, and I don't think that they can truly be valued unless they are earned.
I'm sure this post is scattered and a little jumbled. I apologize for that, and for not being able to focus more while writing this.