Your H is severely depressed. He doesn't care about anything and I would be worried about suicide. At this point, I would go to MIL, not about OW or anything like that, but out of true concern for H and his depression. I could totally see him wanting to assume everything and get a D so that he can feel worse and get to a point where he could end his life. You keep thinking it is all about OW, but maybe he wants to be "free" so he doesn't hurt you by doing something bad to himself.
I would stop focusing your attention on the OW. Yes it hurts, beleive me it hurts. I know I have been there, but with you it is a symptom of depression (mine is just immaturity and selfishness). The more I hear the more I think he needs help.
The only advice I would give (and someone with more experience with depression may want to say if I am wrong or not), but I would say you will think about signing the papers if he will go to IC. Let him know you are concerned about him and his wellfare so you won't sign the papers until you know that he is getting some help from someone and is wanting a D with a clear mind. Once again I am sure this will work, but if you come at it as you are concerned about him and not demanding it, as it sounds the way I put it, but letting him know I care and you are setting a boundary, so I am not going to let you ruin your life could help him get the help he needs and help your M.
Me29 S3 H left 4/1/09 I file 8/2/10 Divorce final 5/17/11 1st http://www.tinyurl.com/25lhu52 2nd http://www.tinyurl.com/2c35ueg 3rd http://www.tinyurl.com/322yk89