Insurance is good, there's options as far as treatment goes. GF is a nurse with adolescent psych experience, and has a lot of contacts in the medical community. We are reaching out to them now.
I'm inclined to treat it as an imminent danger, and act accordingly.
The boyfriend is a tool, and hasn't done much in the way of discouraging this behavior, but I don't know how he feels about it.
She doesn't know that we know any of this, we just found out about an hour ago.
Well I assume the boyfriend is also 15 so he's as useless as the 99% of people here who are also 15 and dumb as fuck. I was just wondering if you can talk to him to see if her behavior seems more like attention gathering shenanigans than real threat.
But honestly, now that I think about it, don't talk to him; he will invariably tell her that you did that, which could light a fuse.
You really have two options at this point:
1) Stay up all night and make sure your family stays safe and get her to a doctor first thing in the morning.
2) Drive straight to an emergency room and have as much proof with you as you can muster up so they don't turn you away. If possible, call ahead so they know you're coming and make sure they know how real the threat is and it's not just you overreacting to something out of context or something like that. You can't afford to be turned away from there because again, that could set off her fuse.
As for how to breach the subject that you're taking your homicidal daughter to the hospital: be kind but firm. She might either fly off the handle go HAM on you or might break down in tears. Either way, stay committed to what you decided and remind her that it's because you love her and want her to be healthy, don't tell her you are afraid of her and want to protect your family (which is probably more true).
Your phrasing should revolve around helping her, not yourself. It's a subtle but crucial difference.