Hi OP. Just caught up with your thread, was wondering how you were doing.
These nights (somebody once described them to me as "the long night of the soul"!) are very testing. They creep up every now and again. Reading your latest posts you've really been on the roller coaster. Great birthday, friends and family over, busy house. Then, children with him, clearing out his stuff, having to hear from the dc about his new home, their part in it, then the inevitable crash for them and you.
You're right. He's a complete bastard. For doing this to you and his children. And it is an active thing, and that's what hurts. He's made choices, and he's lied, because he knows deep down his choices are selfish and cowardly. Be angry, allow the hurt to wash over you. But remember, your dc may have a lovely new bed each in his new flat, but they'll essentially be guests there. With you they'll have a home. Warmth, unconditional love, honesty, a space to be themselves, a future. You can't buy any of that at Ikea.
I think that when it's all moving so fast, everything is changing, these moments always come. Your life before probably went quite smoothly, work, weekends, holidays, ups and downs, but you felt in control. As you rightly say, living with uncertainty is hard, and loss of control equally so. For that reason I understand your need to sort out your mortgage. But if next month he has rent and the mortgage to pay - not your problem. He's moving his life in this direction. You haven't instigated any of this. He could still be living rent free with his parent, but chose not to. Chose. You haven't been given a choice, but you can choose to say to him "your problem, not mine."
Even if he's a bastard he will have to live - forever - with what he's done to you and the dc. Your sleepless nights will pass. Already you don't miss him, for you it's the financials (which will be sorted) and the dc. His dc will grow up to judge him, and he'll be found wanting. He'll never have the relationship with them that he'd like. That's a lot of sleepless nights.
As for making it up with the dc as you go along - that's fine, we're all doing that. Age appropriate honesty, that's my stance. Don't polish his image for them, kids are smart (you know that) they work this stuff out for themselves. Don't tie yourself in knots trying to present an image of him that's totally at odds with what he's actually doing/like as a person. His relationship with them is his responsibility.
Hope sleep comes for you soon. Take care.