Better to stay NC and let her attention move elsewhere or be direct with exBPDgf about the situation and my unavailable-ness? Thoughts? In addition, Is it worth saying anything about the more recent messages to the new gf? IMO, no. Nothing constructive could come from it. So long as I don't pursue anything with exBPDgf, better to just move on. For the record, new gf does know about me and my exBPDgf first contact after one year of being broken up.
Well, you could start with "please do not contact me again" and see how that goes. Telling her about your new relationship is not necessary and won't help. If she keeps trying, you can block and delete messages, don't return them, if she does get a hold of you act bored, keep it short, don't engage emotionally. Borderlines are all about attachments and if she perceives there is still some emotional attachment for you, she will keep trying, assuming she's looking to you to attach, for whatever reason, due to what's going on with her right now. If she's in need of an attachment to help soothe emotions she can't soothe on her own, and you're not delivering that, she will look elsewhere, hopefully sooner rather than later. Take care of you!