TRM, I appreciated what you wrote to Junglist. The thing is a bit of a minefield. Knowing that you & several others, here, have received professional advice on this is reassuring.
I'm probably not going to put this very well, but I want to try and explain how I feel the diagnosis isn't all that important. Sorry if I piss anyone off, I'm writing it too quickly off the top of my head.
Doing an amateur diagnosis can be dangerous and, strictly speaking, isn't altogether necessary. People like this can make you twist your own self out of shape; doubt yourself to the extent that you longer trust your own senses ... So, after years of trying to "understand" - and of failing to get him to understand you - it's a hell of a relief to find there IS something wrong with him (or her)!!!
I think the validation you get from reading others' similar experiences is everything. It helps, enormously, to find you're not alone, you're not "crazy", and this is such a real disorder that it follows patterns you can research! The actual, clinical diagnosis is only needed if you're going to have to prove it in court.
By and large, we're talking about relationships with people whose personality damages those they are close to. The discovery that you're in a relationship which damages you is a hard one, under any circumstances. Having a label to stick on it (and a syndrome of behaviours to read on) is very helpful in facing the situation and in regaining your emotional & psychological independence.
I can't say for certain that the many people who screwed up my own life have NPD. What matters is that I recognise what they did to me. I'm pretty sure my Mum and XH#2 have Asperger's - it looks, sounds & smells a lot like NPD and, since XH#2 was also deliberately dishonest and hurtful, it's reasonable enough for me to think as if he has NPD. I neither know nor care if that is clinically correct. XH#1 was more than averagely narcissistic, and more than averagely controlling & aggressive. There again, then, it's good enough for me to feel he probably has NPD. Same with a few other people.
I wrote this because I sometimes feel people can get too hung up on the 'daignosis'. If somebody's partner is behaving like this; making you feel like that - then they are in a BAD relationship. If the label NPD makes a useful tool for you to understand your situation, then good! Try not to get bogged down in the details - especially not with your partner! What counts is how they act, not how they're labelled.
I really hope someone else will put this more succinctly