Hi WickedStepMum, you're going through so much right now

Parenting/stepparenting teens is tough enough without BPD involved!
I'm guessing from your user name that you're the stepmom? Did I get that right?
Blended family dynamics can make what should be "routine" / "united front" parenting decisions feel high-stakes. I'm a stepmom too, I get it.
Does your daughter (or stepdaughter, please correct me if I'm off) play you and your H off of each other? Are there any other parents involved? Does she make threats like "if you try to make me do XYZ I'm just going to Mom's house/Dad's house/other house"?
I think we're both on the same page that the current setup isn't sustainable. You and your H need to work and sleep.
What do you think is the worst-case scenario that would happen if you and H got whatever involvement (legal, police, whatever) it took to get her on that plane and into residential? I.e., is your biggest fear that she would disown/reject you, hate you, physically hurt you/H, hurt herself, outsmart everything and run away...?
I also have fears about if/how my stepparenting might "blow things up". The kids are 15 & 17 and it's really hard to navigate -- I don't want to be a pushover, yet in order to pick my battles, I let a LOT of things go that maybe I should've addressed.
Keep us posted on how you guys are doing;
kells76