I've been here since the beginning, and OP did have it out with her - over text, but even so.
Let me tell you this - I know this C type (the ex was one of them), and she is loving this drama. By trying to take the high road, you are actually feeding into it. confronting her now is EXACTLY what she wants.
You have made clear to all that it is lies. This is now a smear campaign and the nly thing left to do is to STOP trying to defend yourself and also stop trying to keep these frenemies in your life. They have shown where their loyalty lies, and look - here they are, telling you what is going on and upsetting you.
As I have previously said, the only way to end this is No Contact. With any of them. Tell them why if you like, or tell them nothing and cut them out completely. Do not be breezy at the school gate - blank the bitches like they don't exist. That way, you no longer hear any of this crap, they don't get to feed anything back to C, it's the end of her fun, and it ENDS.
If anyone outside of this bunch of bitches asks you about it, you say, 'I'm sure you've heard their lies, and if you haven't then I'm not telling you - I've cut them out of my life as I'm sick of their drama'. If anyone outside of their group comes up asking you if what they say is true, just laugh (I mean it, LAUGH) and say 'you know what? Consider the source.' This casts shade on whoever told them the lie without you saying anything. If they persist, say 'it's lies, because C is a liar'.
This is tried and tested (by me). It certainly stopped the school gate drama (the abusive ex's family were always there en masse). In the end it was no fun for him to keep up the lies and he moved on to some other poor sap.
And when C turns on K or L tor M, if they dare come to you for help, give them more support than they have given you by telling them to their face that you don''t give a crap. they are nothing but flying monkeys.
Oh, and your ex wants to get involved because he is of the same ilk as C, and he wants to part of the drama. He will only keep it going even longer. Stop it by going No Contact now. Really.