If you look at your relationship as a male/female dynamic rather than a female one and think about her shitty behaviour.
You mention her attempting to gaslight you etc.
This was not a good relationship.
She didn't treat you kindly.
You are now looking at it differently, the scales have fallen from your eyes.
This is similar on a much much lower level of emotional abuse and the same principles can be applied.
By that I mean, you were treated poorly for quite a while, but now she has really over stepped and you are seeing her clearly.
You have clocked her bad behaviour, she knows this, like any person who treats people poorly, she now realises she has overstepped, is back peddling furiously because she really does want to lose you.
You ARE of use to her.
She doesn't want to lose you.
So she will now switch to all supportive and kind, to try and reel you back in again.
This is a similar pattern which abusive, selfish people use in their relationships.
I am so glad that you are so resolutely refusing to engage.
That will infuriate her.
Give her nothing.
To such an extent that if she mentions the wedding, deny it.
Tell her you don't know what she is talking about.
All the time grey rocking her, medium chilling her, being TOTALLY unavailable.
THAT is how you deal with people like her.
If anyone brings it up because she has bitched about you to them, admit nothing and act bewildered, faux sympathetic.
But I am warning you to be on your guard.
She could well turn up on your doorstep full of upset and drama because she is confused, doesn't know why you seem "different", doesn't know what she has done.
Perhaps I am wrong, but be prepared for it.
Do not allow her in your door.
You will get through this and you will be the stronger/wiser for it.