Oh Couch, good to see that you made it through your holiday 😊 I've been thinking about you and checking this thread since yesterday to see how you got on.
It sounds as though your holiday has been therapeutic in a way, I can't tell you how much I admire you for going and for managing to keep things so civil. I was a sobbing mess at the stage you're at, it's my biggest regret that I couldn't even pretend to be calm and together during that time as I really feel I made a fool of myself and I hate to think I gave him that satisfaction.
You know, I think you're going to be ok at the end of all this. It's not an easy road ahead at times but that's what your family and friends - and us here - are for, lean on everybody when you need to. But you can do this 
I well remember that gut-wrenching feeling of dread when it comes to telling the children. All I can offer is to be honest with them, cry with them, don't put on too brave a face as it will help them to see that you're hurting just like they are. The best single piece of advice I had in the aftermath was from my GP. He told me that when I woke up each morning in the days following the children finding out, to do all my crying etc in the shower. Get it out of my system and then put on a positive face for the children at breakfast. I'm not sure if that's right or not but I tried my hardest to do it and it definitely helped me, and I like to think it helped them too. Of course I'd fall apart again most days once I'd dropped them at school but my main focus was to hold it together until they were out of the house.
I did get the defaulted payment sorted thank you, the chap on the phone was shocked that ex had managed to cancel the DD when it was in my name and waived the charge for defaulting bless him. He couldn't guarantee it wouldn't affect my credit rating though
I'm busy now trying to sort out all the forms for the court deadline, I chased CMS yesterday as still hadn't heard anything about the mandatory recalculation only to find out it hadn't even been auctioned! I was then told it's a variation I need to ask for instead (which I did initially only to be told I needed a recalculation grrrrr) and they're now sending out forms. And the onus is on me to prove he is earning more than 10 grand a year!! So, do all you can now to get yourself whatever proof you will need down the line. Tax return if you can lay your hands on it, or, if your H is like my ex and pays himself a wage into his bank then get copies of bank statements. I'm so glad I have at least those as proof. It's such a shitty system and seems almost designed to let the feckless bastards get away with paying virtually nothing.