... .but I realise it is pointless... and that makes me feel helpless... .
Woof,
Let me point out here that you are indeed not helpless. You have identified the situation, came to the conclusion that addressing her accusations would be pointless, and have accordingly chosen not to respond. You have taken action here. The action taken was/is for yourself! That's not helpless my friend. No response is a response, and, for yourself, it keeps you from further engaging in her drama.
You are correct. What I mean is that in the traditional sense I feel helpless... of course, I know this isn't a normal relational situation, so different actions need to be taken... but from the standard standpoint I feel helpless in the sense that I can't respond and engage in a calm and considered manner where we could work things through... We can't have an actual conversation about what is going on/what happened because she doesn't play by the typical rules of engagement... So it's like yes - I have taken action... but this is not how I wanted it to be, and normally these are the actions I would not take! ...