BBNB, I'm really sorry to hear about what you're going through. Sounds awful. I'm in the process of detaching right now but thankfully we live apart so things are easier.
You said about your ex keeping your things hostage:
I'm struggling to see any logic in this
As you probably know by now, this statement sums up what being in a r/s with a pwBPD is like. There is no logic because they are disordered. You'll need to stop expecting this person to act/speak/think logically or the way you do because they simply CAN'T do it.
I used to have these almost funny moments on the phone with my friend when we would be trying to make sense of what my ustbexBPDh had said or done, and then she would stop and say, "Wait. We are thinking logically. That doesn't work here."
It's so very sad that this is how their minds work (how terrible and frightening things must be in there), but unless you can accept and remember it, these actions and words will always hurt and confuse you.
Stay strong and carry on! Keep posting here and ask for help when you need it.