Nanook

I agree with CW, this has been going for way too long. I read back on your threads, very confusing, he is there one minute, then not, he is happy, then not.
I did not read the reason of the separation, why does he want it.

Is he clinically depressed? and is he getting help for it.
He sounds like he wants a wife that just shuts up, and is a piece of furniture, with no opinion.
I have no idea how you have put up with this rollercoaster of a ride for so long. I will applaud you there, you must be the most patient person I have ever read about.
You need to stop this. Like you say why in the new year, why not now. He is just threatening you, because he knows you will back off and shut up, and then you guys will have a happy festive period etc,but for how long, until next time.
Unless he gets a shock and wakes up to reality on what separation will be like, this will keep happening.
Dont be too harsh, By what I read he is a good dad, so joint custody, he gets to spend time with kids reasonable amount of time. No more just popping in etc.He needs to let you know he is coming.
And no more sharing every detail of things with him. If you feel yourself going for the phone, go clean. Let him know what he has to know. If kids want to talk to him, put them on the phone.
This will not be easy, because it has been going on for so long.
But either way, he will get a wake up call, and want you back, and be willing to change, or he will leave for sure, but at least then you will know, and you can at least start a new life.
Reading back on your months, I was confused at times, I was reading the threads, happy holidays, then bad, then house hunting, then at your house, then at his moms.
I will try read your first post as well, just trying to find it, because the ups and downs of your life with him are mind boggling. So just want to try understand a bit more.
Good luck, who knows what will happen, I am sure you think aggghhhh no not again.
You can put a stop to things. Do something.
If you can not shut all doors at once to him, do it gradually, this week say he needs to call B4 visiting, next week, something small again, until gradually, he thinks, hold on this is not nice.
Take care