I hope you didn't read my post as totally negative.
I don't think your marriage is "down the tubes". I DO think there is hope. But only if your husband has a "life altering experience"....right now he is rockin' on with how he has been allowed to do for the past several years. You allow it, Monster allows it.
I was once told by a therapist that for a person to really change, to "wake up and smell the roses" so to speak, that a "life altering experience" needs to occur. I think in this case, that would mean you asking him to leave or you leaving. He has to be given the opportunity to see for himself what life without Deb would be like. I bet he would really miss you.
At this point in time, you cannot worry about him and his finances. He doesn't worry about yours, so stop worrying about his. Worry about Deb and DS. I'll bet the financial advisor can help you figure out a way to make ends meet, and then you should just "go for it".
I think I see that you are afraid if you actually kick him out or leave yourself, that he will run to her and never look back. Possibly he may, but that doesn't mean that he will stay with her. Right now, he only sees life with her thru rose colored glasses. My guess is those glasses may come off real quick once the "thrill" of the affair is gone. JMHO. sometimes the OW needs to be given the opportunity to cook her own goose.
And stop letting him blame YOU and the kids for his obvious mental problems. A man who blames his own children for his need to stray has serious problems. Not nice!
Since he can't seem to make a decision between you and monster, why not make the decision for him? I can promise you, he will respect you more if you do. Marriage can only survive and be healthy between two people who respect each other.