How NPD not fit for Nick? Of course there is lot of people NPD who successful. You may right about destroying it. And in fact Nick almost manage to do that last time around. From Amouranths OF really start to take off until time of phone call was under 2 years. (and she mention in phone call that this was not first rodeo bringing relation to breaking point)
If you read wildkait twitter account (is soapbox of Nick), if you read posts in his leaked instagram account, if you listen to phone call stream. It all point to NPD.
Grandiose sense of self-importance, pre-occupied with fantasies of power and success, requires admiration, entitled, lack of empathy, interpesonally exploitative, arrogant, haughty
It all match.
You confuse his baby rage coming from narcissistic abuse (which is perfect match too for his behaviour, i.e., financial abuse, gaslighting, leveraging, blame shifting, etc.) with BPD.
I don't think BPD match for Amouranth either. HPD seems a good match. But all in all I don't think she have anything wrong with her in sense of mental disorder. Just personality that swings that way. Nick on other hand I am pretty sure is actual basket case.