I think the key here is where you said the mother had her daughter pushed ahead 2 grades. The girl is in the company of older girls that she is not socially mature enough to compete with and interact with on an equal basis. So if she can't get the attention of the girls as an equal, she will get attention from her mother at the expense of the older girls mothers. The girl knows which of her mothers buttons to push to get maximum attention from everyone.
So dry your eyes, your on righteous ground here. You have said often that your a GS leader only because of your dd. If you let this witch get under your skin, then it is your dd that will suffer if you quite. If this witch has pulled stuff so often she as a prior rep. with everyone else, then she's the one with the problems of her own making, NOT you!!