All too often I see people make a very big mistake in a relationship. Not paying attention to a problem until it is too late. There is such a thing as "too late" not all things are fixable.
I would imagine the problems you have had with AH were there before you ever found SR. When we are continuously hurt by another's actions it changes us. How long has it been? From reading your first post I would say 3 years plus. So for 3 years you have been asking, begging, pleading, appeasing, fixing, therapy, marriage counseling , Al Anon, supporting, talking, asking, pleading begging for CHANGE.
And here it is - this week, after all this time - he is presenting himself as the the man you want. It wasn't your asking, begging or pleading, it was an observation that his brother's bachelor life was not what he wanted. Hmmm.
But change is change, an offer to move forward is an offer.
What about you? How have you changed? How has 3 + years of this BS affected you? has it been worth it to finally have him say the things you have asked for that have gone unrecognized after all this time?
How many times I have seen this scenario - maybe not with addiction issues - but a partner try and try and try to fix a problem. And then when they finally put on their running shoes and head for the door the "problem" immediately changes. It's a MIRACLE! Often by that time the person "running" is so checked out of the relationship it can't be fixed.
If you choose to give a go again I hope that you keep your walls up. Keep expectations low and disappointment high. Stay somewhat detached. Protect yourself emotionally. I would have to see a good solid YEAR of dedicated behavior before I would let those walls down.