I can't help you with the NPD thing, as I've only ever heard of it in passing.
Relationships with parents are hard. It sounds like you told him to try to do the right thing by everyone. And I can scarce think of a better way you could have worded those letters. So kudos, and hugs.
I think he's afraid. It sounds like he is afraid that this will turn out badly for you, and he's afraid for you, and thus (by extension) for himself. There are many reasons that a person could fear the consequences of a poly relationship. (I'm not trying to convince you that you shouldn't be doing it, it sounds like it's working quite well for you and you're in a relationship where it will work in the long term . . . I'm just trying to see his viewpoint, to see if I can help you see his response differently.) Maybe he can't imagine a poly relationship working in the long term (especially if his relationship with your mother failed). Living with one other person is hard, living with two is probably harder. Maybe he fears that this will somehow make your daughter become "abnormal" and affect her life negatively in the future. Maybe he is worried that you are secretly unhappy with your decision and want to be convinced out of it. Maybe he's worried for your soul, if he believes that what you're doing is a sin.
It also sounds like he's taking it personally that you're not taking his advice. I can definitely relate to this, as this has always been a theme in my relationship with my father. On the other hand, my dad recognizes that it's just that: advice. It's not a dictum. And even if I do make a mistake, and he did try to warn me about it, everyone has to learn some things for themselves. He knows this, and even says it. So it's not really the same. But I suspect that there is a similarity, which is that he feels he really does know some things better than me (and chances are that some things, he does), and when I don't listen it pains him to see me make a mistake (in his eyes) that was preventable. He's trying to save me from myself. Maybe a little offensive, but I'm trying to learn to see the love behind it.
It is in some ways like coming out as GLBT, and that means you might have to accept that he may be willing to hold out forever for you to realize that this is not really who you are. I have a trans friend whose mother insists on calling him "she". It's not accidental. She also tries to convince him that he was so feminine as a child . . . I think it's because she really loves him and thinks that this is a choice that's going to make him unhappy. He doesn't feel that it's a choice, and also feels that it would make him a lot less unhappy if she could just accept it. But it is as it is, he can't change her reaction just like she can't change the fact that he's trans.
I think the only thing that you can do is allow him to see (maybe try to convince him to look so he can see?) that it IS working for you. There's no guarantee he ever will, and if he does it might take a very long time. It's possible that 20 years of a stable, happy, poly relationship would not strike him as sufficient evidence that it was an acceptable life path. And if that's the case, there's certainly nothing you can do to change that for him. It sounds like he is trying to express his love but also set up boundaries . . . (please don't take this the wrong way, I'm still just trying to see where he's coming from) like you would do with someone who has an addiction that they have told you they don't want to get rid of. Maybe if you imagine it that way - if you had said all the same things, but substituted "heroin" for "poly relationship", would his reaction have been the same? Would it have been appropriate in that case? Because chances are pretty good that he considers poly relationships to be just as scary and dangerous and self-destructive as heroin. Again, I can't make this clear enough, that's not my opinion. But it might be his. If you look at it from that perspective, can you see the basis of love that (probably) underlies his words?
Not that I think any of that necessarily makes it any easier to actually DO anything about his reaction. But it might help it not be quite so hurtful, I hope.