I'm going to go slightly against the grain here. You've had some amazing advice on here, but not everyone's situation is the same as yours. I agree that you need a good lawyer, and I also think that you need to keep your own counsel and not involve your mother in law in what you're thinking.
I was going to say that you need to talk, but you're now doing that this evening, which I think is a good thing. I didn't understand why last night you kept wondering when he was coming home - I thought you would have been ringing or texting him asking where he was and saying he needed to get home and talk to you. He is your husband even if he's not behaving like he is. Especially as at the moment your daughter is away - it seemed a good opportunity to get things out in the open and shout and scream at him if you wanted to. You say your daughter is having counselling and this could be devastating for her. I know you have told her, but that doesn't mean it's sunk in.
He seems to have completely checked out of the marriage, and whilst it's utterly wrong that he is having an affair, it does sound as if things weren't right anyway. You have obviously spoken to his mother about his detachment.
You've mentioned his mother and friends, but very little about your own. Do you have real life support? We're a bunch of strangers on the internet and it's great to be able to come on here and tell us what's happening and get support but it's no substitute for real people. You are a SAHM with a 15 year old - in theory you should have time on your hands - do you have hobbies, interests, friends? They will be invaluable to you. I guess I'm a bit concerned - and I may have got this wrong - that your marriage is your world. It was the comments about your daughter's boyfriend being a 'keeper' and looking out for her that made me concerned. A mum shouldn't be wanting her 15 year old to find a 'keeper' and be dependent on a man in any way. She has her whole life ahead of her.
I really wish you well and hope that your talk tonight goes well. The only thing that matters is that you get what is right for you. You are worth so much more than a man who cheats on you. He is treating you really badly; I've known people have affairs but I've never known anyone to have the bare faced cheek to put the bills for the affair on the joint account that their wife manages. That's contemptible.
If you do want to save your marriage, and that's something you need to decide inside yourself then talk it through and lay down the law. Don't let him treat you like this. Im not saying you should by the way!! But you must do what is right for you. No-one else. Only the right thing for you.
Good luck. Be brave x