You are absolutely right – there is no conclusive evidence about the cause of BPD. What researchers have been able to do is draw some correlational data, which is unsatisfying at best.
I’ve always tried to avoid the word “cured” when talking about Psychological disorders. A personality disorder is certainly not a disease one can get cured from, but it CAN be treated and people CAN definitely get better.
It’s true, Personality disorders tend to require long-term treatment, since their very nature affects time-persistent personality traits.
New studies are being conducted every day and I have faith that with improved methodology, well-documented cases and neuroscience we can soon have more conclusive evidence.
I should probably do a more current literature review and post my findings.

I visited your blog. You have good insights. Best of luck in your studies!