Op, you sound like a strong character on the whole, and I think you can get you and your kids through this.
Reading all of the posts the alarm bells for me were:
Changing his name via deed poll on the day of the marriage - why weren't you able to take his surname, who's idea was it for him to do this, and what surname do you have?
Not having his name on the mortgage - what reason did he give you for this? I'd be suspicious that he had another mortgage somewhere else with someone else, or a honk load of debt from his other lives.
Everything done through your bank account - so he has no paper trail in your life. What names are on the DC birth certificates?
He's a low earner - have you actually seen any pay statements, or transactions. Chances are the CSA may be taking a lot of his salary, or he's trying to avoid them altogether with name changes etc.
He has other kids along the way - I'd seriously be checking he has left them along the way, and isn't still convincing them he's a darling husband or partner either. Are you in contact with the other mums? Might be worth doing so to find out a little more about him.
His parents were abusive and are old school - really? The truth or just his word for it. His dad married a protestant, so he can't be that old school can he? Even if she converted to Islam she still has a protestant family and background wouldn't she? You only have his word that they were violent - I think he's done everything he can to keep you away from them and in fear of them.
He has left? Did he find somewhere quickly? Is it somewhere you can go and visit to talk, or is he cagey about that too?
Your wedding day - your friend had to be best man, not one person who had apparently been out celebrating with him on his engagement could stand next to him in church, not one understanding cousin, or school friend could be there.
I do know muslims who live a secret life from their parents, but they nearly always have another family member they can confide in, of their peer group - cousins, siblings etc, it surprises me that no-one knows about you, which would strongly suggest that he does have a separate life with his family, and at worst case, a wife and kids.
Op don't be harsh on yourself, writing it all down in black and white it sounds horrific and you'll be wondering how you could have been such a dullard - I felt the same. You're not - this man areshole has been dripfeeding you utter crap for years, whilst you've been having kids, setting up a home, working to keep a roof etc and generally been distracted with a busy life.
I have forgiven my partner for some utter selfish stupidity in the past, but this man doesn't know where real life begins and his lies end. You need to know answers but I am afraid that they will only end in utter heartache. You cannot trust one word he says. You need to protect yourself and you dcs.
I know it sounds harsh but I am hoping you actually aren't married, so that you can walk away without another bucketload of hassle.
Please see a solicitor or CAB and find out your rights and if searches can be done. I would consider ringing the police if I had suspicions.
Please get yourself an account sorted in your own name and make sure he doesn't have access. It's very hard to close a joint account without partner agreement, but you can redirect your money to a private account.
Please do a joint credit search on Experian if you can, it will list all his aliases and other accounts. It costs about £12 but could be totally worth it. If he's lying about all of this he could have loans in your name..cars in your name etc. Don't panic, just get yourself a search asap for peace of mind.
As for your house - well it's yours isn't it, so deepclean it and get new bedding and a mattress topper, move the furniture and bleach all the surfaces if it makes you feel better that he is expunged from the place. Don't blame her though, she may be completely innocent with all his excuses.
Do you know if she is pregnant? Can you contact her for some info?
Look after your DC's, it's a very sad time for you all, but you deserve respect and this man has given you none.