Get the biggest, heaviest frying pan you have in your kitchen and tell him he leaves before the morning or you will wrap it around his ears for him.
Then tart packing his things. Big bin bags will do.
You are married to a scummy, awful man who deserves to sleep on a park bench tonight and for the foreseeable future. He can figure out the rest of his miserable life for himself. Do not waste one minute worrying about him or listening to his pathetic explanations of what has been going on. No, there is no depression here. Just a philandering, pathetic, vain ego-tripper who thinks nothing of using his own vulnerable child to salve his conscience. Toss him out, and all his belongings. He will be fine.
I suspect there is no impotence either btw. There is no such thing as a man who gets used to being impotent and for whom it isn't a problem; this is the age of viagra after all.
This is the absolute worst case scenario for you. Please don't let yourself be manipulated by worries about your DD or DS and especially about the H into accepting things as they stand.
"Yesterday he gave me an assurance he definitely wouldn't leave for 6 weeks. When I moaned at that he said 6 months. I feel I have a reprieve." The gobsmacking nerve of this man.... Whose child is the 2 year old? Is it his?
Call your DD's doctor and all her support services asap and tell them what has happened and how your poor DD has been shamelessly used over the past while by this utterly selfish, immature, abusive waste of space. She will need help to get over this. You must get this rotten man out of the DCs' lives, and soon. How could he do this to your poor DD...

Go to your CAB, or talk with a solicitor ASAP.
You can do this on your own. You can live a good life with your DCs. You can be supermum. You can get through this.
I am so sad you are facing all of this, especially with Christmas ahead.