But truly want to end this ... .diffuse it ... .
I can understand the frustration with your efforts at this point not working. I'd like to echo
Once Removed what have you tried? That being said, you already know that rescuing is not a good idea with pwBPD I'd like to extend that an say that rescuing is not generally good with non's to. Try not to put this on yourself or make it your duty to put out fires if they're not listening don't diffuse it ignore it eventually they'll understand that you're not interested in your ex.
I get confused ... .Feeling that although I want closure for him as well as myself ... .I have to be the one to provide that closure for him ... .and compromise myself in doing so ... .So as to also provide some relief to those I care about who are trying to rescue him ... .Very daunting ...
Yes very daunting it's not your responsibility to manage other people's feelings - feelings are about self management, don't rescue!
You don't have to provide closure for him he's escalating threats of harm, shift the focus away from them and want they need and focus on what you need by taking care of yourself eventually it will feel less daunting and you'll feel like you have less stress and more energy.