Um...there are w.a.y too many 'what if's' in all of this to really consider this a serious relationship. He needs to man up and do and be what he says. No B.S.
You Deserve Better.
Alcoholism is not something you go Toward in life. Watching someone like this, go in and out, up & down, round & round, is just ...hard. Really hard. I see this pattern in what you said: he draws you in, then spits you out. Over & over.
All your questions need to go unanswered for now. I know you care what happens to this guy, but I think, he has a lot to do before you should try to figure him or his reasons out. Alcoholics are emotional & irrational. Take it Extra Slow. For your sake, make some distance and give him time to get it together.

I speak from experience. I speak as a person who took on that person, supported & loved, and gave & gave & gave. I believed he wanted to change. He did Want to. But it is a really hard fight. It is a disease. It can be fatal.
Again, respectfully, You Deserve Better.
Believe that. It's True.