Hi op
I'm so sorry you have had such an awful time with this group. It is so hurtful when people who you have invested time and emotion with turn out not to have your back and be who you thought they were. It makes you question yourself and your judgement.
Keep going and keep strong for you DD and just accept the 'friendship' for what it was at the time, helping you through your marriage break up but now move on, knowing are not good people and in the long run it will be better that you didn't invest anymore time in them.
There was post a while ago that said there will probably be lots of much nicer mums in the playground who may have been out off by you being in the clique and now may be more open/value a friendship. Often the less attractive, less 'exciting' looking mums will make the better friends in the ling run.
Re C and her DH, I don't think her DH will have a clue. Don't think for a moment she through him out, it was telling that during that time she wouldn't accept visitors as she was too upset. That's because he was still there. When she had the others round for cocktails on the Friday, he was probably out after work. She went from too upset to see people to having a few round for cocktails and children for a sleepover. Who organises a sleepover days after their marriage falls apart. She made up the whole story to deflect from the questions that M might have asked after her DH said you weren't facebook friends and it worked.
The others all stick with her as she entertains, arranges social events etc and they don't want to be excluded from such social gatherings and siding with you would have risked all that. It's got nothing to do with who they believe, they just can't bear to lose their social status.
You may never know why C turned on you, jealousy of your friendship with others in the group probably and unfortunately as a single mum with limited spare cash you probably don't fit in as well into her clique. I lost more than 50% social invites once I became single, some people only want couples at social events. Hurtful and narrow minded but true. I was immediately dropped by my closest fried when my DH walked out, just at a time when I most needed a friend. Still no idea why but I think I just didn't fit the bill anymore.
I think ignoring them is the best course of action for your DD. If you confront them they could start to stop their children playing with your DD overnight. That said I would gradually start encouraging your DD to have other friends, start inviting new children round to play.
Good luck up. [Flowers]