“If you're in a relationship with someone who presents traits of BPD, is there a way to have conversation about it that doesn't backfire?”

I’m not an expert on it either, and wasn’t exactly diagnosed BPD. Maybe others want to weigh in.

IMHO, I certainly want to know if I or what I might have and am very open to discussion. I have found it’s the more narcissistic people who don’t want to hear any criticism.