Well there are always things on both sides of any long term R where one person misbehaves. But you can't move forward with one person not being willing to let go of the past and forgive and look to the future. That's regardless of any abuse. This 'debt' is basically him hanging onto this resentment. You can't make him let go of that, but you won't have a decent M without him letting go of it. His resentment may be valid, but my H also behaved pretty intolerably at times during our M, and I've realised that the only way to R is for me to forgive him for that, understand that (as you said) it wasn't done out of malice and let the past stay in the past.
You can only control your side of this, which right now means protecting yourself from his unacceptable behaviour. Please don't let him blame you for anything work related from him moving out! And maybe it's time to talk to youngest and decide on a different sort of party? If he wants youngest to see his family let him arrange to go to them, you can keep out of it. I agree that for the time being you should keep communication via text or email, it sounds like talking in person or on the phone is just getting him upset and attacking you, not very productive.
