Ah Jaysus - you're doing great at the grey rock thing, keep it up.
Avoid any temptation to message them - I strongly believe, reading the updates, that K is at least as bad as C, if not worse, and that you messaging them would make fuck all difference, only provide entertainment for them all.
It would appear that the backbiting has been going on for a while behind closed doors, and C's first mistake message has just brought it all out. I believe that M, K and the other one (D?) were maybe just keeping a foot in both camps to see what you would say/do, but they were never on your side.
I think it's important to take note of Contrary's post about the shit trickling down to the children - but also that that isn't a given, it very much depends on the children themselves. But just in case, keep up the idea of broadening your DD's social circle so that if they do start to turn against her, she has other friends, both inside and outside of school, to turn to.
Your exH may have done you a favour, may not - but there is no point at all in trying to find out, or having any contact with that poisonous bunch of harpies. C's H may or may not be having an affair, but that is not your business or circus - apart from a sense of "ha! serves you right" if it comes out, you have no need to know what he's doing. Or what she's doing.
Grey Rock it all the way, lady - you're doing the right thing. Reduce the drama until they get bored and fuck off and leave you alone.