Ok, I have read through every thing and I have seen this behavior before. This girl is a psychopath. Not in the general use of the term, but in the clinical/medical use of the term. She is a conniving, antisocial animal that cares nothing for other individuals and is highly self serving. I have seen a few of these.
The mother has a very hard time giving up on her hell spawn. Her mother has been manipulated for many years. And she wants the best for her child. She probably thinks much of her daughters behavior is her fault or the fault of the divorce. Its not. psychopaths are born not made. Its like the mother is a dog who has bore a wolf.
This women who now calls herself Fierceton will hide behind her good looks and intelligence for the rest of her life, manipulating those around her in ways they will not expect. Mostly because she will go to further extremes. She will scheme for a long while and those around her will give her the benefit of the doubt at first not knowing what she is capable of.
I am sure that Fierceton will leave in her wake a series of lies and ex-friends that will speak very poorly of her.
She is the quintessential ‘sociopath’.
My brother was one.
The part you wrote about the mother is exactly the same as it was in our house..........
Her diagnosis could also be “Destructive Narcissist”. A very common problem with young adults who were spoiled growing up and then turn into self centered, ungrateful, gaslighting monsters when they get out on their own.