OP, having read the entire thread, I have to say that in your shoes I would still divorce this man. He is a liar, cannot be trusted with money, and one day you might even find the police at your door, which will be when you find out what this was really all about. You still don't really know what happened, do you? You only know what he's told you, and we all know that he's lied. He even lied to you about where he was going, the night when he went round there, and spoke to the woman and her DH. Why didn't he tell you where he was going?
I just COULD NOT trust him after this.
Also, I think you've been asked quite a few times, but haven't answered, have you now taken every possible precaution to ensure that if you do stay with him, (in my view you'd be stupid if you did), that he can NEVER get his hands on any large amounts of money, ever again, without your signature? Have you checked your credit to ensure that he's not taken out another loan, in order to 'pay back' what this woman owed you? I would also, ensure that you're registered with the Land Registry, so that if anyone tries to sell your home without your knowledge, you will be informed. If you haven't, you can do it here: https://propertyalert.landregistry.gov.uk
Finally, have you spoken to his sister since his visit, and told her what he's done? It does seem VERY strange that he felt the need to go and visit her, after lying to the woman about some of the money being hers, as there was clearly no need to do that. Is there any way in which his sister could have loaned him the £40k, to pay you back? Maybe he called her and told her he needed a large loan for something, and would call in and explain it all at the weekend, but could she lend him the money NOW, as he needs it URGENTLY! Sorry if this sounds unlikely, but after everything he's done, quite frankly, I wouldn't believe a word he says, hence my advice to divorce him.