Hello Solost - long time no post - I've been away and have spent last night and this morning catching up with both old and new threads (detail and all - I'm in Yorkshire too, btw - it's a big place though!). Sending you a gruff, MN style hug this horrible morning.
I have to say that having read more than a week's posts in one go, I'm not tremendously surprised to see the turn this has taken - although like WWIFN, I expected that he'd take another tack, to deflect attention with a financial crisis or something which made it 'a bad idea to tell the kids now, we don't know WHAT will happen'.
Started writing this before the latest phonecall this morning, and am just editing the MASSIVE (!) post I've written urging you to tell them - I'm so glad, despite the way it's come about now, that that's what you're doing. Thank God. It WILL get better from here on in. Some of what I'd written might still help, maybe.
As an outsider, it is crystal clear that he has been manipulating you. But, I would say don't feel bad that you can't (couldn't?) see it in quite the same way. Your position is unique in that you know him better than anyone else, and yet are also now in a position that makes you the one most in the dark - as it is YOU that his manipulation is focused on. I can imagine that this must be one of the most difficult parts of being the betrayed partner. All your very complex, long-learned patterns of knowing and relating to this man have been ripped up. A bit like trying to operate an upgraded version of a program you know inside out. You keep pressing the wrong buttons, but it's so automatic, you can't help carrying on pressing them 
Wrt the situation right now, it doesn't really matter to what degree his 'breakdown' was 'real'. As someone said further upthread, what's happening is that he's had a massive reality check, really for the first time since he left. In HIS eyes (though really, this is nonsense) the children knowing means that It Is Real, It Is Permanent. And all of a sudden, Mr. Love-Of-My-Life, Bye-Bye-Wife-And-Family, Sorry-Kids! is a snivelling, shivering wreck. That tells you all you need to know about how his fantasy new life has worked out. He's made the biggest mistake of his life, and he knows it. Cruel fate, eh? The latest phonecall - I think you're REALLY now seeing a glimpse of the anger which shows what a nasty, controlling little silent bully he is. He knows he's come to the end of the line with managing to control your response to what he's done. It's amazing when you look back that he did manage it for so long - what an actor! - but basically up til now, when your parenting instincts tell you that it has to stop, he's dictated the way the family THAT HE LEFT should manage living without him (how: well naturally in a manner that portrays him in the best light possible! He can't be the baddie, can he? He's perfect!)
When I say that his perception of the children knowing as being the 'endpoint' is nonsense, what I mean is that he can't see that that point has already been reached. This hasn't all been happening in a vacuum. Their Dad hasn't been there since August. They will have known and understood FAR FAR more of your state of mind than you may think. You know that they are now being damaged by the stress of the situation. The fact is that, whatever happens in the future, EVEN if you got back together, this situation is happening to THEIR FAMILY right now, and they need an explanation for it, and to be able to talk to the adults they trust, and start processing it. I'm so glad you're telling them. You know that this is the case, and that their welfare should come first. We don't even need to discuss his opinion on this - someone who uses as their ultimate threat that they 'will never see the children again' - won't ever understand the real parent's desire to put the child's need ahead of their own. As a father, those words should never have come out of his mouth. More shame for him there, than for all the weeks of betrayal with BB put together. Don't forget those words, should you ever start thinking that the family could come back together. That's who this man really is, Solost.
What might happen now?
- This totally ABSURD Eastenders-style cliffhanger moment that he has whipped up will, well, evaporate. The children almost certainly won't turn their backs on him - but there WILL be a massive sense of relief for both you and they, and you can sit down with them (and maybe PILs too) and start working out what kind of Christmas you'd like to have, and what happens from here on in. So he's not going to be around - good. Who knows what will happen in the future. He'll certainly change his mind on a daily basis, by the looks of it. Focus on the children and PIL, and be thankful that the cause of all this isn't going to show his sulky, FUCKING UNWELCOME face at Christmas.
- Mr. Histrionics will get what he sorely needs, whatever the reality of his mental state at the moment - a massive, teeth-cracking SLAP around the chops. He doesn't get to DECIDE whether this is reality, THIS IS REALITY! Yes, you chose this. No, it couldn't stay as a fantasy, because incase you haven't noticed, you've laid waste to your entire family by what you've done. If he's simply trying to manipulate and control you, it's the best thing that could happen. If he's genuinely stopped coping now that the path he's chosen looks rocky, it's also the best thing that could happen, because this limbo can't continue. This morning's phonecall of course suggests the latter! He's better now, then, is he? Breakdown's over and he's back to aggressive shouting, blaming, feeling sorry for himself? (The man who left for another woman!!!)
Note that WWIFN was right - no suicide attmept for our Mr. Solost. His form has rather been to choose to hurt other people instead of himself, hasn't it though? Long-term threats such as not seeing the children EVER AGAIN! (gasp) are just silly, as is all this whining about control. Well yes, when you abandon your children, they do rather become the responsibility of the parent left with sole care of them, no? And as others have said, if he does ultimately choose to distance himself from them, they're much better off without him.
Finally (Thank God, I hear you cry)- if you are tempted to feel sorry for him in this state, to rationalise this morning's phonecall - just stop and think. The feeling of total fear, disbelief that your world is crumbling around you, panic as to what you can do to hold on to what you want - remember when YOU felt that way? That's right, back in August. When he told you that he'd met someone else, and was leaving you with your three children, to manage on your own, thanks for the memories, bye. What did you do? Threaten him that if he left YOU'D leave and never see the children again? Smash things up to try and frighten him into staying? No. You acted like a parent and a mature person in the face of the most terrible shock. What a difference between the two of you, now that his moment for fear has come. Never forget that he saw you in this position of fear and dread, and what did he do...?
...walked away.
Yes he is despicable. He put the phone down? Bonus. Leave it down. You don't have anything to gain from speaking to him. Great news about him not being there for Christmas. When you do speak to him, make sure he knows that now you know he's willing to hurt the children to control the situation between you, that all bets are off - he'll get no more special dispensation from you. That you want a divorce and formalisation of finances as soon as possible, and that you look forward to you and the children moving on without him and beig HAPPY.