Hey victoriousmama, welcome to the group!

Lots of family members here in the same boat with you. You'll find moms, dads, stepparents, grandparents... the whole range. While I'm usually over on the family law board (it's my husband's kids' mom who has BPD type behaviors), I couldn't help but be drawn in by your post.
It's frustrating, almost like trying to make someone hold a soapy bowling ball, to try to "make" a pwBPD (person with BPD) own the truth about something. We've experienced that with the kids' mom. It's such a no-win scenario... you keep trying to "get them to see", they keep coming up with ways to avoid taking responsibility.
And yet, there's this huge opportunity with your daughter.
She vehemently asserts that she can't remember, and has no idea how this stuff gets here?
Wow... oh my gosh... if that's true... that's VERY serious. VERY, VERY serious. How could she disagree with the fact that memory loss at 17 is incredibly serious?
You can say that if the memory loss continues, then you will definitely take her to a specialist AND a therapist, because you care deeply about her, and if she is saying she has blackouts and memory loss, then you'd do anything to figure out the cause of such a high-risk, sudden-onset syndrome.
She couldn't object to that, right?
After all... either the memory loss is serious enough that unknown persons can approach her and put stuff in her bag, and she has NO IDEA what happened...
or it isn't a big deal?
Hmmm... seems like a double bind!
Would be interested to hear your thoughts, and I'm sure more members (especially parents) will be along soon.
In the meantime, again, welcome!
-kells76