I am a recovered addict and a loved one of a now recovered addict and I can tell you that his response does not show a willingness to make the life changes necessary to not only save himself but also to save his marriage in my experience.
Deflecting is common with addicts and it shows how immature and how self centered they are. You confront the issue that is pulling your family apart and basically attempts to give you an ultimatum back. That is not what someone who just decided to change his whole life around would come out and say.
My mom just told me a story the other day of a friend who found out her husband is cheating on her. When she confronted him, he literally says "I can't believe this is happening to me". SERIOUSLY? Wouldn't you think the first thing he would say is "I'm sorry...I'm sorry while you were sick at home with cancer and also dealing with you father dieing from brain cancer I was having an affair" but no.
It is the same thing. He is only capable of worrying about himself and protecting his addiction. I am happy you have a plan for if he doesn't step up to the plate. And while you don't want to upset the children, they will one day realize that you did not have a choice and that it was the best for them if you do decide to leave. Also, he is the one that ruined the marriage, not you. Your kids are lucky to have a role model that is strong, independent, and has the self respect to say enough is enough and mean it.