@HeadLikeSpaghetti
I'm so sorry this has happened to you. It has to be a terrible shock. I admire how you've handled everything. Telling your mother-in-law was exactly the right thing to do. You warned her about his threat of self-harm, and she knows why you've made him leave.
The biggest concern I've seen mentioned is whether or not to tell your children.
As an early teen, I knew why my parents' marriage was ending. I never had to wonder why she made him leave. I think knowing helped the four of us (kids & mom) grow stronger as our family changed. My sweet, trusting, accomodating mother transformed into a warrior. She protected us, loved us fiercely and we knew we were the most important things in her life. I learned that strong women don't accept that kind of treartment. I never blamed her. She wasn't the one to destroy our family.
You've done nothing wrong. It's all on him. The damage began the first time he went looking. He knew the chance he was taking, but continued for years. Even if you'd never found out, he was still hurting the family All that time and energy focused on hiding his secrets, not to mention the risks he was exposing you all to.
A close friend's husband was arrested during a raid on a massage parlor. All his dirty little secrets were exposed, but somehow they managed to hide all of it from their young adult children. He was a "good" father and when she divorced him, her kids blamed her for destroying their family. They had so much sympathy for their father and none for her. Three years later, after his 2nd wife left, she finally told them because they still expected her to reconcile. It was his shameful behavior, both parents lied to them, but they still blame their mother.
I know you'll think carefully about what to tell them, but your kids are old enough that they'll know something is very wrong. Especially if they haven't seen a lot of fights and arguments over the years. They don't have to know all the details but they need an explanation.
It's a harmful, destructive secret, but you can't protect them forever. There's a good chance they'll find out later, and the lies will hurt them all over again. Remember, you didn't do this. He did. Family counseling may help, and you'll be there for them every step of the way. Your lives will change, but you'll get through it together.
I've included some links to resources with (hopefully) helpful information about what to tell teenagers. But if you can, get professional guidance on how to tell them.
Sending you hugs and strength!
www.psychologytoday.com/us/blog/surviving-infidelity/201305/should-the-children-know-youve-had-affair
beyondaffairs.com/2013-archives/tell-your-children-about-the-affair/