Absolon,
You write extremely well, and it reminds me of another who has sadly passed on. The emotion is very evident.
You have conducted yourself extremely well, and you seem to be navigating yourself out of Infidelity.
There is not much I can advise, as it has already been posted by other members.
Am posting only to see if I can help give you a possible reason for your WF's behaviour. This is in no way condoning her behaviour, but if it helps you understand it, it may help you heal. BTW, understanding does not mean agreeing.
From what you have posted, and the key is in the meeting with your (STBX?)-FIL, it looks like your WF has been an Entitled Princess/Prince (EP) all her life.
With EPs, they do not live in reality as their world has been a huge game to them. If they fuck something up, Daddy/Mommy will sort it out.
Crashed the car? Daddy/Mommy will buy you a new one, or it will be repaired by them. The EP does not have to lift a finger to solve problems that they have caused.
So, by crying to her parents, she is getting her parents to 'fix' the problem she caused in her 'game' with you and the AP. She is hoping that Daddy will give you a job to 'sort it out', or Mommy will guilt you into staying. EP will not have to lift a finger.
To EPs, life is a game. Consequences are not 'real', so do not have to be considered in the whole scheme of things, as someone else will clean up the mess.
Unless she snaps out of this 'entitled' mindset, she will never be safe for you.
How do I know this? Well, I know of a few people like this, and would even consider myself a marginal ex-EP. Luckily I woke up from that world, and am actually enjoying life more, as it is more fulfilling.
It sounds weird, but being accountable for your actions is quite empowering. This is what EPs lack, a sense of purpose/accountability. This warps their worldview, and they become insulated from reality.
So, all in all, the behaviour your WF has been exhibiting is not to be condoned, but I do hope that this might help you understand a possible thought process.