Sorry you are having such a hellish day. He's an arsehole.
All the posters are right. This is him up the ante to get you back into line. It is that relationship dynamic again. He is attempting to control you as usual, and punish you for getting on with your life so brilliantly.
Easy to say but resolve NOT to engage with him like this again. He has, and will continue to do what he wants anyhow. He has no intention of seriously engaging with you re this or anything else, and you're right mediation will be pointless. He'll introduce her when he wants.
And although you can't bear the thought of it, you'll cope, and so will the DC. It won't ruin their lives forever, you'll see to that. In the longer term it may well add to the poor opinion they'll have of their DF but sadly nothing you can do about that! In some ways when she does finally appear it will be a relief for you, one less thin to worry about, and one less area of control for him.
You will get through this. You will feel better. You are all going to be better without his toxicity in your house. And when your DC tell you he is marrying OW/some other poor soul you'll smile and say "That's nice". And not give a fuck!