I just looked on this site to try and get help for my situation and found your post. Wow, narcissists heh!!! I separated from my narc almost 3 years ago. We have 4 children, 1 with special needs and we are only now attempting to settle financially. He has stopped providing any child support (I think he feels I get enough money from my part time job and Centrelink) and he has the children reluctantly for 4 days a month. However, he constantly puts on Facebook how much he loves the kids, how hard it is with a special needs child and how greedy and awful I am (because I ask for child support money).
I have done so much research on narcissism and narcissistic abuse that I ended up signing up for a psych degree and have enough data to write a book (may do that one day). Your ex sounds quite text book with the control, lying and cheating. What I have learnt from my own experience and from research is that the only way to deal with them is to be completely dismissive and have no unnecessary contact with them. Do not look them up on Facebook, do not respond to text messages or emails (unless strictly about the children) & just don't react. They will continue to fire venom at you and throw in these 'red herrings' but please try to see it for what it is. They have lost control of you and do not cope with that type of rejection.
All of this is exhausting I know. I am using a free legal service at the moment as I know that my ex will drag this out as long as possible. Is this something you could do - it takes a little pressure off if you are not having to fork out each time their story changes. Their whole point is to wear you down - when I feel like this, I just look up a couple of narcissism sites to be reminded that this is not my fault. Trusting your husband (or wife) with finances and fidelity is expected of marriage so please do not be hard on yourself and don't blame yourself.
I hope that you continue to post on here about your journey. You will get through this eventually but I will keep checking in and support you on the way.
And in all of this try to remember - they have to live with themselves ALL the time. Imagine how awful that would be!!