This sounds like a living nightmare. Small community, abroad, false accusations, people taking sides. I fear there will be no way to get out of this unscathed.
Honestly, right now I would be taking the high road. Protect your kids, yourself and your DH and do what’s in your best interests. If that means losing your friends, so be it.
I would contact the school preemptively, tell them what has happened (the incident itself, not the social drama) and that the only reason you’re contacting them about something that happened outside of school is because DS may manifest inexplicable behaviour to them (upset, worry etc). Be very clear about what your DC have told you. Don’t gossip, don’t talk about “and if she approaches you pls don’t believe her etc”. They can work that out themselves if they are bothered; the main thing is your DS stays stable at school.
As for your group of friends, leave them to it. If they’re prepared to believe the queen bee over your word which you haven’t even had a chance to explain, it shows them for what they are. Know that it’s only a matter of time before they too are on the receiving end of something. Carve your own path, leave them to each other.
I don’t know what use a lawyer would be in this situation. Frankly I think an appropriately timed, steely, tiger mom “you ever do anything to even remotely affect either of my children again, you will find the matter taken out of my hands and into official hands” might be enough.
From what little you’ve said of this woman - overbearing dickhead arrogant DH, she quick to jump to conclusions, DD possibly exposed to more mature behaviour than you’d consider normal in your world - it’s possible the Home life is not a-ok. There’s something not quite right there. I would be wary in any event.
The incident in the park sounds troubling, I too would have been upset as the girl’s mum. But I still would not have jumped to conclusions about this incident and have done anything re the party (she’s moved way too fast, even though I’m sure she started the conversation with the host along the “I don’t think we can come on Sunday”, “why not?”, “well xyz happened”, “oh but queen bee that’s not your fault! Why should you decline! I need you to remain in my social circle! You come and I’ll uninvite other friend” lines).
Gosh I do feel for you. What a terrible situation. I’m glad you and DH are on the same page and can stand firm together to protect your kids.