@GilbertMarkham - I do have skin in the game and here’s how it went which is why I am very sceptical of this situation all round.
Immediately after we split up, she was introduced out of the home as a friend. Then the popping in and going for walks began
This takes us to roughly the point OP is at right now or will be in a few weeks.
Three months after he moved out he asked her to drop the children back to me.
She pushed for this as she “wanted to be part of his whole life”
He pushed for this because he thought she was amazing and such a good person (in the care profession)
Meanwhile my kids are sleeping in my bed every night for nearly a year trying to come to terms with their dad being gone.
Let me pause here to highlight the last point. Their dad was chaotic. Trying to get divorced, moving house, accommodating a new relationship, work - even if he saw the kids, it was rarely alone without her and it was all Disneyland and Facebook posts but he just wasn’t truly present.
With her quickly being referred to as stepmum and being in charge of the bulk of the parenting, often without him being there, things quickly turned sour and I’d be getting texts from her about things I should do different or hadn’t done to her expectations. Read that again.
Meanwhile, when I introduced someone a full two years after we split, the kids would get interrogated about the most minute details about this man.
At no point through all this did I slag off the dad or her to the kids. She’s welcome to him. I played nice at the door drops, I tried to smooth waters when the kids were angry at him or her. Or just the situation in general. Sadly, in the process of getting her shoehorned into their lives, they were not able to be as considerate and have on several occasions voiced to the kids that I did this or didn’t do that.
Roll on to my kids being teens, they have a shitty and volatile relationship with their dad, don’t fully trust her or him as they are not consistent, and sadly one of them put two and two together and worked out that their relationship started as an affair. I don’t know if one of their friends told them or if they just did the maths. I got asked about this in the car - great. One of the comments leading up to the big question was “she just got introduced and it all happened so fast”. Again, left to me to manage those emotions as any pushback to my ex and the step mum is met with narcissistic ridicule and gaslighting.
In a separate about death where my kids asked what happens if I died, I asked them as part of that conversation what they thought would happen if their dad died. I am honestly a little shocked as I thought they were closer after all these years but they both laughed and replied that “she’s off”.
There is zero doubt that my ex has used the stepmum responsibilities as a means to and control his relationship with her at various points of the relationship. It is also important for him and many other men to “tick the boxes” and show the world their success as a man. Married by 30, millionaire by 35 etc etc. This ramps up even more if the man in question is a narcissist which I am not saying the OP’s ex is. But he is a man.
OP you must be exhausted from this thread and I wish you nothing but the best as you navigate the way forward. Be the constant, be the light, and don’t forget yourself in all this.