Welcome to the site. Very sorry to hear about your H. But very happy to hear about your recent breakthrough of dawning insight. That's the first and most important step. - We're all here for each other to the best of our abilities. Great support is to be found here from people in like situations and I think you'll find a massive recognition and positive mirroring for your current troubles.
" .. he physically threatened me, demeaned me, denied it, threatened divorce, blamed me and others, and then 72 hours later acted as though everything was normal."
I think that before you consider confronting him further, making demands on him to seek professional help and the like, you should make sure that you are physically safe and not alone in this. That you have the back-up of close friends and family. It may at first feel shameful and wrong to reach out and tell the straight dope to those closest to you - I know I had a very hard time overcoming my vanity and pride in that matter. But you must put your own, and your kids' safety first, then consider his needs. BPDs can go very, very bad when they feel threathened. And the biggest threat to them is losing their part ego-nourisher, part victim - you.
"Here is my question: how do I get him to see that he needs counseling, that he needs help and support, that he deserves it and that something is wrong with his mental health? That he can have a happier and more satisfying existence? How long might this process take? How patient must I be?
Also: should I tell his parents my suspicians? They see it and are subject to the behaviors also, but not as intimately as I do since they live far away. "
As others have stated, also in my experience, it's very hard getting them to see that they have a complex of problems that needs professional treatment. Their disease is rooted in their very core-being; one could say I think, that they 'don't have a disease, they are the disease'. They may have a reasonable measure of raw intelligence, but their emotional maturity level - the ability to deal with the causes, meaning and implications of their own feelings - may be that of pre-pubescent children. Getting them to the point where they realize that they have go to work on their personal emotional habitus is key to any progress, I think.