Hi Mr bbbs, welcome to the group

If BPD is in play in a relationship, there can be a lot of confusion and questions, just like you're experiencing. You're not alone in trying to make sense of what happened.
BPD is a real and serious condition. It's more than just a personality style (which is flexible), it's a personality disorder (which is a pretty rigid and less functional way to go through life). We can't diagnose your ex here -- we're peers, not professionals -- but what I can say is whatever is going on, whatever it would be called, wasn't a relationship that worked.
pwBPD (persons with BPD) often make choices based on their intense, overwhelming, and wildly varying inner emotions. Managing those out-of-control emotions is often their top focus. That means that they may say and do things that "don't make sense" to us externally... but do make sense to them based on what they're feeling.
It may not be possible to explain every detail of exactly why she did what she did on social media, or why she still has some of your things. It could be BPD tendencies (she's doing those things to manage her intense emotions)... it could also be part of a normal messy breakup. Breakups happen to everyone and are rarely clean. The "breakup with satisfying closure" is more rare than common, I suspect (I think I got meaningful closure from 1 out of 4 breakups, all with "generally normal" partners); it'd be even rarer when BPD might be in play.
All that to say -- if it's closure you are seeking, you may need to provide that for yourself, by getting support for yourself here, and/or with a therapist or counselor. Feel free to talk through things here and learn more about yourself and
why you were attracted to her; that's ultimately where you'll get the most clarity

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Has anything changed with her having your stuff, over the last week or so?