As far as I know, G.Docs has a history version where you can see the changes. It can be an evidence. Next time, always screenshot your post along with the update date.

And I am wandering, why don't you confront that cheater herself? You have proof, you can tell her to stop. I know it might spark a new drama and she was backed up with the boss, but you're not wrong and this must be stopped. Just be careful though.

For me, it seems like the boss tries to make you quit, by making the workplace feels like hell. That's why she closes her eyes despite the cheating. She won't fire you because of that five months pay. At least, I think it'll give you a little bargaining power. It just my speculation though.