I have begun to wonder if instead of being BP she is simply a spoiled brat. I keep getting the sense that she is acting even when she is being nice.
She may very well be, pwBPD have little sense of self and often are role playing their entire live. When "doing the right thing" it is often what they believe is appropriate, it doesn't come from the same natural state of balance which is the essence of self that others have.
BPD is a permanent state of processing things differently, not just a series of episodes. It is just that same sides of it cause grief and others dont. Hence the "good" often has a sense of the inappropriate about it, 'over the top/ try hard" if you like. Lots of parroting and mirroring to fit in.