This is the most difficult stuff. You have a lot of variables going on; the fact that she's 15 is not a small thing.
Going back to your original question, of if D15 has BPD, have you read Blaise Aguirre's book
Borderline Personality Disorder in Adolescents yet? My takeaway was that understanding the function of the behaviors is important, as many impulsive yet "developmentally normal" teen behaviors appear on the surface like BPD behaviors. What
function is doing X (reckless driving, drug use, backtalk, sneaking out, etc) serving? He also talks about your question -- is it BPD or not -- and makes some parenting recommendations either way. My sense is that it's not like chemotherapy, where if you don't have cancer, it'll really hurt you. Parenting a child "as if" the child has BPD likely won't make things worse if the child doesn't. Unfortunately the book deals with situations where both parents are on the same page about the approach; it doesn't touch on when parents are divorced and the child has BPD-type issues.
The other book I thought of for your situation is
Combating Cult Mind Control: The Guide to Protection, Rescue and Recovery from Destructive Cults by Steven Hassan. I'm not saying that having a BPD parent is equivalent to being in a cult necessarily. I think what I'm getting at is you have so many moving parts going on, and each person you speak to or book you read or site you visit "kind of" helps with part of your situation, but nothing really helps with everything. You may find that reading outside of BPD literature could give you perspective on an approach to try (or a piece of an approach to try) with D15 that could inch you forward.
I haven't read Hassan's book in a few years but the preview jogged my memory -- that his approach isn't convincing others that they're seeing things wrong, it's saying "Look, you're telling me that you're making these decisions out of your own free will, that you're thinking for yourself, and that nobody is pressuring you. OK, so then there shouldn't be a problem with us talking about your beliefs? If I point something out to you, then you can handle talking about it and thinking about it, right? If your beliefs are sound, then there won't be a problem with applying this decision-making toolkit to them." It's a respectful approach that "assumes" the person has curiosity and research skills.
He steers away from the "deprogramming" or "intervention" approaches, and has tried to build a legal, ethical, and respectful approach to intervening with loved ones totally taken in by harmful groups.
Interestingly, it strikes me as similar to Dr. Xavier Amador's book
I am Not Sick I Don't Need Help!, in that both books emphasize that fighting delusion head-on through logical argument, force, coercion, and convincing, aren't effective.
While neither of those is a solution on its own, maybe you can roll some of their ideas into your approach. Both books can be fast reads; I'd recommend taking a look if you haven't yet.
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I can say that even for us -- where SD18 from age ~6 had told H "you're not my family", would defend Mom, would blame Dad, would use Mom's language of "you left
us", would refuse what minimal parenting time and overnights H had, and seemed 110% sucked into the "shared family mythos [delusion]" between Mom and Stepdad -- SD18 is now open with us that she does not like the environment there, if SD15 wasn't there she probably wouldn't be either, she wishes Mom and Stepdad would be actual adults parenting, and that it's high conflict over there. I'm not saying "sit back and do nothing, it'll work out" -- SD18 and SD15 will have a lot of pain to work through -- but more to say that even us, fumbling our way through, with minimal time with the kids, with minimal avenues to point out the toxicity, with no custody or decision making, and with 180 degree different cultural beliefs, have seen SD18 start to come back around.
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Has D15 texted anything else telling you not to talk to her T?
Did you let the T know that you'd read other material on alienation?
And is D15 still doing parenting time with you?
