I agree with Vapo and strongly advise taking the higher road on this. I never once engaged with OW on anything and I have no regrets about that. The thing is, many OPs love the drama and I wouldn't feed into that at all.
Vapo is right about karma. My view on karma is that it is a natural unfolding of events based on your earlier behaviour and choices. Example - you choose to get involved with a married man. He leaves his wife. Both of you then get to be in a relationship with someone who cheats (rather than tries to save his marriage) and who is willing to get involved with a married man. When life becomes stormy, as lives so often do, they aren't likely to traverse those seas well.
I would take a stance of - hey if you want to have an affair and leave our marriage, go. Be robust and specific. You have lost her and you will need to start dealing with that grief and GAL to start filling the void that she left. Whether she turns and wants to come back is up to her, but that is unlikely to happen in the short term and she would have some work to do if she wants back in at some point.
I think you are bang on the money with the finances. Your W didn't have a good role model, which is a shame and make sure you take any reasonable protective steps to look after important assets. I think that is a good example to set for your girls.
As for her and OM. I once read that that only something like 3% of affairs actually transition into longer term relationships - after 7 years (I know that's a longish time!) 97% of those relationships had failed....who knows what may happen going forwards.
Self-focus and forward motion are what you need here and some private grief and anger time too. Plus be the rock for your girls. Be minimally available to your W and only for kid stuff and leave yucky OM to wallow in his own pond. He is beneath your notice and if it wasn't him, it would just have been someone else if she was inclined to stray.
Best wishes - this may be a hard year for you, but you can make it hard AND good.
T 13 M 7 Me 48 H 46 SS 15 BD 7.14 PA D final 5.16 (H filed)
We receive & we lose, and must try to achieve gratitude & embrace with whole hearts whatever of life that remains after the losses - Dubus
