Hey Kats3, welcome to the boards

you sound like you're at a tipping point with your D21's behaviors, so we're glad you reached out for some support as you navigate how you want to move forward.
It really stood out to me -- it's interesting that your D21 shares with you that her friends think she has a drinking problem. Do you think your D would agree?
Sometimes I'm curious if a person with BPD-type traits and behaviors, while perhaps resistant to straightforward intervention like residential or DBT, might accept "yes, I have a problem in this other area of life", and be okay with getting help for a peripheral issue. While joining AA or going to rehab/detox won't fix BPD, it could be a foot in the door to her being open to treatment. Anyway, just a thought.
How has she done in school so far (as she's 21, I'm guessing she's done some college already)?
It must be so difficult to be in a position where she's technically an adult, yet you still have say over some parts of her life (college tuition, room/board, etc), and she does come home, but tells you so many painful things about her life.
Would you say that the THC use is your biggest concern right now?
Keep us posted on your journey -- we'll be here to listen.
-kells76