Oh Zig! First big hugs to you! We will always encounter things that force us to grow and good for you for examining yourself with all the questions posed to you on the boards.
My thoughts for whatever they are worth.
I agree with KD a million times over in all you need to do is own your choices. It's what I discovered when I stopped hearing myself owned my past and simply said I regret those things but it's not who I'm going to be. Your responsibility ends with you. It doesn't extend to MIL, FIL, SIL, her GF's kid, H, OW or H'a parenting of S. you are on a picnic blanket of one.

As hard as it is to accept we can't control everything our kids experience or how it will affect them. IE we can decide how and when we want to have the talk with our kids and you never you know when they'll go to a friend's house and get told in a totally different way about the birds and bees. And no latter how hard we try to be the perfect parents they may take one insignificant detail and decide that was a very good or very bad experience. All you can do is come to the understanding with yourself that everyday you will be the best person and mom you can be and later on not look back and judge yourself with older more experienced eyes with hindsight. You do the best you can at the time. No guilt!

I carried loads of guilt about my parenting. S17 asked me a question and I told him an honest answer about how hard I had to work as a teen mom in university and he said oh it doesn't matter I don't remember. How many times did I cry myself to sleep when I was younger? Wonder if I was doing the right thing? Be filled with heartache because I'd see him for an hour a day between work and school and he shrugs it off. And my coworker says the other day "you two have such a close relationship. Anyone can see that" awwwwwww tears!!!!

So don't worry 3 steps ahead about how you or H will deal with telling S about H's affair or when his relationship with OW began. That's unnecessary drama!
Last thought you know the best thing about being separated? You're not obligated to have all this interactions with ILs. She is wrong to have any conversation with you about your M and to discuss any convo she had with H with you. You're right there is a pattern but it's not your pattern do don't take it on. If I were you and feel free to tell me to eff off LOL, I would not talk about H, your M, or parenting with her. You've already said you don't agree with her choices in parenting or M so don't let her influence you in yours. If she brings up what H tells her, cut her off. He may not know she's telling you this...you certainly don't know if she's telling you the whole convo or how she wants to spin it and it just sets you back.
You can't influence an outcome. And you can't control anyone else but you. So whether not S agree with this whole time in your life when he's 40 isn't something you can control. Sadly you can't control how H conducts his relationships. Don't worry about something that hasn't happened. Wanting to run away, take S and run? This is crazy talk not picnic Zig. Remember you told someone else that their attitude influenced their kids? Funny that S is now talking about magical Seattle island isn't it?

(sorry for typos on my iPhone)