I feel for you OP but do think your husband's passive (if somewhat understandable) attitude is feeding the problem.
Unlike some others, I think it is ok to come on here and vent annonymously, and if she is a manipulative hysteric then it is ok to say that. I don't think she automatically deserves respect regardless of how she behaves. Just because she is a mother doesn't mean she is a good one. There are plenty of bads ones as we can see from some of the threads on here (e.g. see the Stately Homes thread) It might be politic to act civilly in front of her, but that is different. I don't believe there is a sisterhood (and the ex obviously doesn't think so either!).
But it sounds like your husband never really learned how to deal with her effectively, and was inclined to give in to her which of course only feeds the crazy behaviour in the long-run. Some of it might have been with the best of intentions, like he was trying stop arguments, or trying to avoid putting the children in the middle of an argument, or trying to avoid telling his children their mother is telling lies about you, but really he needs to find some way of standing up for you now (and it should be easier now that they are adults, in terms of they should be better able to handle it and be more mature, but maybe harder for your OH to do because of years of habit of not doing it).
To some of the other posters, I don't think it is entirely fair to call the husband an idiot over his behaviour (even if his behaviour can be seen as somewhat stupid) as he probably got worn down over the years, and conditioned into giving in to the extent that he may not even see how much he is doing it. And how many women do we see posting on here, perfectly articulate sensible women, getting manipulated by men, or the same man, in the same ways over and over again? It happens for a mixture of reasons but also because some people are just very good at being manipulative, getting their own way, getting money out of people and so on. Sometimes it takes quite a few times for the person to get burned before they learn how to see it coming and to stop it in its tracks.
The husband may have been a victim of emotional and verbal abuse as well (if the ex was crazy enough) but may not even be aware of what this is, or how to deal with it.
But OP I think your husband needs to change his behaviour, but he might need advice on how to approach things with his kids. You might also need counselling or someone to talk to so you can vent in a safe place and also talk things through with someone. I am going to PM you a link to an internet group you might be interested in (I am not sure if it is relevant to your situation but it might be).