Thanks all. This is all helpful.
In terms of whether she may have a crush on me...and whether she may have had a crush on the previous person who filled my role...I think it's a remote possibility, but unlikely. Unless she's bi! She talks to me about guys she has crushes on and how she kind of wishes guys would pay more attention to her, etc. One of the more normal teenage/less disturbing things about her. She also has moments of self-awareness for sure, though they are just moments. She says stuff like "I'd want a guy just to hold me, but I wouldn't want to sleep with him, I'm too messed up for that." Poor thing.
As a matter of fact - she met a guy I was friends with who I had a thing for at the time (a bunch of us went out for dinner together) and based on chatting with him a bit, developed a crush on him! Which she subsequently told me a lot about, without actually knowing that I liked him. And she's had a couple of rock star encounters (CD signings etc) after which she's been quite giggly and dreamy about the guys. Honestly, to me it is all much more that she's trying to adopt my life, than that she has a crush on me. Liking a guy I liked (possibly she picked up on some vibe on my side anyway), teenage crushes on rock stars but they are bands that I like and indirectly introduced her to...etc. She's adopted a lot of bands, books etc that I like, but will also get mad and say I'm trying to control her or that she "wants her own life" if I suggest a band she might like, or something.
Apparently BPD can, among other things, result in confusion over which way your sexual orientation lies. So it's possible. But among other things, she has zero relationship with her mom. And her brother has kind of disappeared from her life. I think it's more likely she wants substitute parents, siblings etc.
I agree that she is young to be diagnosed at this point. I think it's around this age it's supposed to really emerge, but they also want to see evidence that the symptoms last for at least a year. I just pray she can hang in with the therapy. Even if it starts off with them looking at her for depression, I suppose they will be able to figure out if she needs to be referred on or whatever.