I am not sure what to say, what a horrible place to be in, You may never get all the answers you want. Obviously he has some issues. the worse break ups is when its very much one sided.
I would recommend you might want to seek counselling, you will go through a lot of emotions, and I am sure it will take time. You cant false him to see his kids, the most important thing to do, is be there for the kids, try not to **** him off, don’t lie to them, its ok not to always have all the answers.
Sometimes not knowing everything can be actually less hurtful.
My partner left me after 4 yrs, within a month was see someone else, I was broken, devastated, angry, hurt, as worse of all hatted she could leave my son so easy, after we had discussed what would ever happen she should keep seeing him, especially as he had gone through so much in the last 3 years. But the only time I could move on, was when I excepted I was never going to get all the answers.
I hope this helps a little