I am a fixer. I can't get out of my own way. I have been working real hard on that. But right now I'm in full on fixer mode. I'm trying to shut it down.
What steps do I take?
As my mechanic likes to say - "now thars yer problem".
The only thing you should be fixing is yourself. Everything you describe about your life - your wife is a mess, you take care of everything, you do everything, you fix everything. I think that is why you are in this entire situation. You absolutely should read 'No More Mister Nice Guy' - its short and you can find it free - because like I used to be, you are a posterboy for a controlling, fixer, issuing silent contracts and holding resentment when she doesnt hold up her end of the bargain.
As for what actions to take - what you should not do is take any actions that have anything to do with your WW. Continue to detach and force yourself to accept that your WW is not yours to fix, and she never was whether she is having an affair or not. However you can enforce boundaries of what you will and will not tolerate. By blowing up her affair, you showed that you will not tolerate her affair any more. But you cannot make your WW work on the marriage. You cannot force her to decide to come back to you. You cant even force her not to go back to OM. She is going to be completely insane for the foreseeable future, any energy you expend on her will be wasted at best, and backfire at worst. And if I were you, I would never entertain a conversation with her about her OM problems ever again.
However, there are plenty of actions you should be taking, and they are all about you and your kids. Continue to work on yourself - read that book! Go to the gym - lift heavy weights. Go to some meetups for something you have never done. Plan fun activities with the kids. Lock up your finances, get investments away from her and all her family, and do everything you need to be in an extremely strong position should you end up in the divorce process.
Just continue to be awesome and trust that everything will work out as it should, because it will. Either your WW will come crawling back when she is out of other options, or you will decide you can do better for you and your kids. And my money is on the firmly on the latter - I have dealt with divorces for a lot of guys just like you, and to a man 1 year later they are happier than ever and simply cannot understand why they ever put up with their exes.
