Honestly OP I couldn't handle that. Jesus he sounds 'altered'. So all of this kicked off because your eldest DS didn't gush over 'D'H when they returned from the GP's
your poor DS. He's not old enough to play these passive aggressive, emotionally 'adult' games.
As far as your DS is concerned your DH bullies him and scapegoats him whenever there's a problem. I can imagine DH makes everything about your DS. Your DS, although unable to understand the complexities of his stepfathers angst towards him - which is probably jealousy - will feel uneasy, insecure, low, uncomfortable, unwanted and blamed. He probably does act out and behaves like a little bugger but he's trying to figure out where he stands as his position in your family is unclear to him at the moment.
It might help for his own father were made aware (lightheartedly say that he and SD are not getting along) so that he can spoil him a little bit - he deserves it. Pull your ex up on his "busyness" if needs be. His children need him more than ever at the moment.
I would absolutely feel I had no choice whatsoever to ask your H to move back to his mums. I wouldn't do anymore therapy with him, he's a narcissist. I'd arrange weekend visits for his two DC's immediately as he's packing his bags. I wouldn't arrange it for your first two, what's th point? I'd tell him I have no choice, that living together is damaging everyone's happiness and you can't slow it to carry on.
Meanwhile I would enlist all the help you can get, your parents sound fab (my DM is on her knees after a morning with my two!) friends that can help with school runs. Throw a few bin liners in for a service wash if you can (sometimes that is like receiving a diamond to me).
Then I would bear down and make it through the next few months. It will be hard but your stronger than you know and above all of this you deserve some peace. 