I think A seems like she has a lot of build up anger and frustration with you she hasn't known how to deal with. I don't know if it can be due to the fact that you have BPD and you haven't been able to be as strong as she needed you to be. Despite that, it does sound like her problems go beyond BPD. Perhaps she suffers from NPD as well which makes it hard for her to feel empathy for you (and her own child), especially if she begrudges you. I think it is awesome that despite your depression and your BPD you have found the strength to take care of her child and I hope he turns out well.
One thing I can come up with is that I think A really needs boundaries. You need to set boundaries when she crosses them and not let her walk all over you. I think the fact that you don't may want her to provoke or hurt you even more just to get a reaction other than the one she expects. To me it sounds exactly like what a child would do to a parent who doesn't set limits. She cannot respect you either if she doesn't see that you protect yourself. You just have to be careful and not let her get to you and remember she is suffering too and take a distance to her. That will teach her that she can't get away with everything and that you really want to feel better and care to do so. And for your grandchild, I think it is important that he sees you stick up for yourself so that he may learn to do the same.
It sounds like despite all the pain and rejection you luckily have something that makes you see the light in the dark. I really hope you keep on being this strong, and remember, take good care of yourself, don't let people treat you like dirt.