Hi Andrew30,
Tough situation
She accuses you of cheating, and you have a lot of female friends. This is a party she isn't invited too. It sounds ripe for conflict!
Having a several hour discussion about the topic seems pretty long given you have a line in the sand about this topic (potential cheating). She may be realizing that she can keep you engaged in the topic even if she can't change your mind (negative engagement).
From a few months ago:
I made it very clear to my wife that any further accusations of infadelity would result in me leaving her for good (and given I'd already moved out for a few days and ended our relationship - she believed this would happen). She has kept to this boundary ever since.
Are her conversations mainly focused on you cheating? If so, then it sounds like this boundary (more of an ultimatum, actually) has lost some of its teeth. That would compel her to keep working the topic over because the consequence is soft.
Setting all that aside, the thing that jumps out the most is the comment about her viewpoint being extreme. People with BPD tend to feel deeply inadequate, have a fundamentally hard time with trust, and struggle with executive control -- so pointing out that her views are wrong/extreme, that she is damaged, is only likely to make her feel even more like she is not good enough for you and you'll find someone else. Cycle continues.
Do you validate her feelings in these conversations? It's hard for lots of people when their partners have relationships with the opposite sex, or go to parties that they aren't invited to. Maybe in addition to validation, try SET skills (support, empathy, truth) and then be willing to reassure her (in limited ways) leading up to the event, and the ways in which you are willing to check in with her while at the event, if any.