I did come to the conclusion that ultimately a diagnosis (formal or armchair) really didn't matter. It was all beside the point. The fact is that he displayed those characteristics and that behaviour, and therefore the effect on me was surely the same/similar.
As I said, I saw BPD characteristics in him. I recognised the characteristics only because I'd briefly dated someone in my 20s who had been diagnosed formally with BPD, and some of my ex's behaviours were so similar. Obviously that doesn't mean he had a PD, but like I said, it felt similar. The way his behaviour made me feel was familiar.
Not long after I noticed he had these similar traits, he attempted suicide and as I sat with him in A&E, two psychiatrists interviewed him with me there and were asking him questions that clearly suggested they were thinking along those lines (again, doesn't mean a diagnosis, but they obviously clocked the behaviours). I split up with him soon after so I don't know what transpired in the long-run, but after the psychiatrists had seen him he was referred for DBT counselling. My own psychologist, when I went to her for help in processing the trauma of the relationship, agreed that he "ticked a lot of the boxes" and that from our interactions she commented that she saw some "quite narcissistic" behaviour too.
Again, none of this is a diagnosis and I'm quite prepared to accept he was just a really difficult individual, but the point is that the diagnosis really doesn't matter. If his behaviour was so much like that of a disordered individual that not only me but several MHPs saw the same kinds of behaviours, then the effect on me would surely have been the same as if he did have that PD. And so finding out how I could heal from a relationship with someone who had a PD would surely put me on the right track. My psychologist suggested I read a book that she often recommended to people who had grown up with a BPD parent, as she said it contained a lot of information to help move on from that kind of relationship.
I think this was key, in fact. I wasn't hand-wringing, trying to analyse his behaviour and figure out if he had a PD or not. I mean, I had certainly done that in the past, but when I split up with him I stopped doing that. Instead I started just finding resources that were aimed at people who had been in relationships with disordered individuals, and work out how best to move on. In other words, I shifted focus from him onto me. I think it was the focussing on (and working on) myself that made the biggest difference, but I wouldn't have found those resources if I hadn't taken the possibility of a personality disorder into consideration.