Regarding the kids, my experience and attitude has been to let them figure it out by themselves. D7 is asking questions every now and then. I'm not lying, but I'm not criticizing either. I cannot criticize their mom anyway: it's their mom and I've read over and over again how hurtful it is for kids to be torn between the two parents, to have a conflict of loyalty. If anything, when they finally figure out what happened, they'll likely have a renewed sense of respect for my self-control over the whole thing. So I would say given that your WAH's behaviour is so out of norm, just let them come to their own conclusion over time, it will be way more convincing than if you try to bring them to it.
I have hope that you'll find the strength to focus on you, on your life and ignore his antics. That you'll get to a place where it doesn't matter what's the name of his girlfriend, what's her living arrangement, who he slept with and when. It's liberating and it frees up a lot of energy for the things that matter to you.
M39 D6 D3 (at S) S 2014-09 D 2016-09
"You can't start a fire sitting around, crying over a broken heart" - Bruce Springsteen.
