Does your D15 accept her dx? I'm so sorry to hear that she had to be Baker acted. That had to be quite a ride for the whole family, although like you say, now there is a dx and DBT therapy, as well as a community of people who understand what you're going through. Glad you're here.
If you feel comfortable sharing, what took place for her to be Baker acted? Did the dx come during the first one?
Her version of extremely upset is probably a hundred times more intense that what we experience. I come from a very reserved family and had a major revelation when one day I experienced intense emotions -- made me realize how hard these kids work to manage emotions. It's no walk in the park. A lot of cognitive functions go offline.
Is she applying any of the DBT skills she's learning successfully? We focus on tiny little changes here

Small counts.
Also, in my area, DBT for teens involves 4 modules, so it takes roughly a year to get through the whole program.
I do think it's helpful when family members learn DBT skills and read how to create a validating environment and use skills that improve interpersonal communication and relationships.
Have you found books that help? BPD in Adolescence by Blaise Aguirre is excellent and so is Loving Someone with BPD by Shari Manning. I also found When Your Daughter Has BPD by Lobel to be useful in a different way -- he focuses on the family structure which can be very useful.