I know what I would do at this stage, given being silently dignified is failing ( she is continuing to blacken your name) it’s too late to kill her with kindness that horse has bolted, and neither your joint friends or her weak sap of a husband have the backbone to stand up to her. The legal route will feed the drama, and may not work.
There is only one nuclear option left to you; and we all need to be ready for this ( and be vigilant on the thread):
You post an announcement about the situation on the class or year Facebook page/What’s app page or you individually email every single parent that is likely to hear her malicious lies.
You state the lies that C has spread about you. You explain that the messages have nothing to do with you. You confirm the conversation you had with her husband in front of named witnesses M and others.
You state that at no point will you ever find this behaviour acceptable and that it now your wish to clear your name, and ask people to respect your wishes by stopping the slander in its tracks.
It is possible it will become a legal issue in the near future, and you would thank them to protect themselves by remaining uninvolved.
Attach all three threads into the message. So they can read for themselves exactly what has happened to you.
Any reasonable person/ parent will be absolutely horrified at your treatment. Horrified.
You will shut down the lies.
You will expose C to scrutiny.
You may win some support (one would hope)
You fight fire with fire now.
By Monday she will have sent out all sorts of flying monkeys to say you are trying to wreck her marriage and her life. She will escalate this next week. Crying at the gate, gathering sympathy and support from absolutely everyone. She will play the broken victim, and you will be presented as a man eating monster. She will isolate you with her lies. I fear she has only just started with this campaign to really hurt and discredit you, and she won’t stop until you, your creditability and dd are in the gutter, broken.
You have to stop her, and you do this with the truth for all to read.
You have nothing left to lose now.
Expose her in full, let others see her as we all do. She will wish to god she had never done this, when she realises 1000s of posts have your back. What else can you do?