I am a daughter of two alcoholics, as well as a recovering alcoholic myself, with 18 years of sobriety. I'm also a BS.
Regarding your WH's relapse, you're not doing him any favors by keeping him from his true bottom. I'd bring on the consequences.
What boundaries have you set for him, and which of them did he break with his latest shenanigan? What are the consequences of him crossing the line? I worry that there are none, while you and your MC try to figure out what to do with him next. Is he participating in R at all?
Are you in AlAnon? If not, you should be. I worry that you're trying to "fix" him, and the most basic truth is, you can't.
He has to do this on his own, with a good sponsor and a ton of honesty and determination to not allow this disease to kill him. Alcoholics will tell themselves (and anyone that will listen to them) that they don't have alcoholism "that bad" and will remain in denial that they have a potentially fatal disease. They'll half-ass recovery until they realize that this disease is coming after them like a hound straight out of hell, trying to kill them. They don't get to that point until they've been hit squarely in the face by a lot of consequences. THEN they get serious enough to do the hard work that the 12 steps demand of them.
Don't be his soft place to land so that he can convince himself that what he did wasn't "that bad."
If I could say something to him, from one drunk to another, I'd tell him this: Sorry buddy, but it's time to shape up or ship out. Get serious about getting your addiction and your life under control. Right now, you're tossing away the best chance you've got in life. Don't blow it. Your wife may be there to love you and make things easier on you right now, but your addiction still wants you dead.
Oh, and the diagnosis of sexual addiction must be made by a professional, so none of us will be able to answer that questions for you. Sorry.
Remember to take good care of YOU and try not to be too co-dependent with him. Your MC is right, this is another D-Day, so you have some self-care to do right now and when your head is clear, some decisions to make. I wish you peace and serenity!