- you do NOT look like a twat, you're the only one in this situation with any class.
- do NOT let your exH get involved. C's husband may not know anything, he may, but it's really NOT worth going around C like this.
- do NOT bother talking to C about this, definitely do NOT contact her or go round to her house. Stay well away from her - she's dangerous. She is a lying psychopath who will never ever back down, she will gaslight you and everyone else around you, she probably even believes her own lies by now. You will NEVER win against her.
I know injustice stings, trust me. But trying to put the record straight almost never works - the only thing that works is TIME and the others realising that hey, actually, you weren't the problem at all, it was C who is as mad as a box of frogs (I don't mean clinically insane either)
If the rumours start to affect you beyond the school gate then a cease and desist letter might be necessary, even though your belt has to be tight - maybe, if your ex is so keen to help, he would fund this for you. You have to remember as well that he is still with the OW, who would doubtless LOVE all this drama - you don't need to involve him personally beyond a handout for a solicitor.
Other than that, you just have to keep away from them. I agree with not defriending them on FB, don't fuel their drama - but you can Hide them instead, and you can adjust your own settings so they can't see yours unless you do a public post. I have friends who are Acquaintances only - and most of my posts show only to Friends Except Acquaintances. They only get to see Public posts, or the very rare Friends Only posts.
The Whatsapp group - again, don't leave it. Just leave it be and see what happens. I almost never use WhatsApp but have a few convos on there in case people want to contact me using it.
And then just maintain a civil and cool attitude towards them - you are better than them, you are a grown up and they are just silly immature women who thrive on drama - show them that they can't get to you.