Oh Lost 
I've just read your updated posts and I'm open-mouthed in horror. Not so much at what you've discovered re the porn and the continuing contact - par for the course as far as I'm concerned. Not to trivialise that but if he needs the "thrill" an ow gives him, then he needs the mental stimulation of porn for those times when neither you nor ow(s) will provide that thrill.
It's his behaviour that's shocked me. Sorry to rant but how dare he put his hands on you again. How dare he - he's the one so obviously in the wrong and you're the one who's trying so desperately to keep your family together. His misplaced anger is part of the "script" but his aggressive behaviour towards you is most definitely not. Please please don't go on holiday with him - you'll be in a confined space away from all you are familiar with and I have this gut feeling he will use it to his advantage to corner you into not only believing his shyte but accepting that you are strictly on a "need to know" basis and no more.
I know you want to do it for the DC - believe me, I know that feeling that it must be worth some hard work to try to fix this. But this is more than hard work. You're sick with the stress of it and he's doing nothing to make that go away - nothing. He's trying to bully you into accepting his idea of marriage and it seems to me that that goes against the grain of everything you believe in. After all, it's only a few short weeks ago that you were in a bubble of happiness where you believed that you had your soul-mate and you could trust him with your life. You don't want this - you mustn't stop fighting for yourself and your own peace of mind. But I am worried what will happen when he eventually realises that he can't bully you into submission.
Lost - you must take certain steps. Even if you can salvage your relationship, you will only make him take you seriously if you take steps to end this awful awful situation he is putting you and the kids in.
Firstly, you must put together an overnight bag with some of yours and the kids things in there and keep it at your Mum's. Put the birth certificates, passports, red books along with a change of clothes (nappies, formula, spare bottle etc) for all of you and keep it there.
Secondly, make an appointment to see a family solicitor. Don't be scared of your current financial situation - solicitors have seen worse believe me. Make sure you put aside your latest mortgage statement, latest bank statement, marriage certificate and any docs re: child benefit and tax credits. If you can find it a copy of his latest accounts and/or tax return as well as any pension and savings docs in either of your names.
Thirdly, consider reporting him. Even if you ring the local domestic violence unit and explain that you'd like to log an incident and for them to put a red flag on your address in case you need to make an urgent phonecall. I know this is the one piece of advice you won't want to follow. I know that. But you must take it seriously otherwise he will not and he will keep on pushing and pushing until you can't control him. He put his hands on you in front of your children - on multiple occasions. Show him you mean business.
Lost my lovey - please please look after yourself. I'm so glad that you've decided to ask your Mum to come. You sound as if you're trying to deal with this alone and you shouldn't. You can't. Your body is buckling under the stress of it all and your relationship with your children is suffering. You can't shield them from your husband's stupidity. That's his job, not yours. All you can do is make yourself a safe environment where your physical and mental health aren't pushed to their limits on a daily basis. Stop trying to save him - he's a big boy and he has to live with the consequences of his actions. Save yourself darling.
On a slightly lighter note (!) I remember your mentioning down-thread that his first wife was a nightmare. Was she really? Do you know this for sure or was it reported second hand via him? I'd put money on him being bitter and twisted about his first wife waking up to the fact that he was a wanker. It could be that she wasn't quite so nice about it when she found him out [pathetic attempt at humour I know!]