Hi Maresa,
Welcome and hello

Multiple diagnoses make it so confusing. Have you read Blaise Aguirre's book about BPD in Adolescence? It is a puzzle to me why clinicians are reluctant to make diagnoses when they're young. When you live with BPD teen it sure seems clear, but I guess the doctors don't quite see the whole picture and don't want to rush it.
Buddha and the Borderline tells a story about a young woman who was actually dx'd BPD as a teen, but no one told her. She didn't find out until a decade later and asked to see her mental health records. Reading her story makes you realize how hard we have to advocate for good treatment. It's hard, too, being the mom. Who knows what the treatment team is thinking
NC is so painful. Could she be using it as a way to regulate her feelings? It's often the people closest that trigger the strongest emotional reactions. Sometimes, NC can be a way for them to regulate their emotions, lacking other skills to keep themselves steady.
My SO is a physician and is on a team that deals with high-conflict patients, many of them are people with PDs. By all accounts, he feels very capable working with them and diffusing situations. And yet he feels so turned around when it comes to dealing with his BPD ex wife and uBPD/bipolar D19. I think it's easier to problem-solve and see things clearly when our own emotions aren't in play.
Do you have other kids? When do you expect D15 to come home?