I feel bad for him and simultaneously pissed off, and I don't know what to do aside from keeping on truckin'.
Stay off his case. To put this in perspective, I was and still to a degree am to my family what your brother is to your's: the black sheep. Everyone "concerned" about me. Badgering me over my life choices. Telling me how bad I'm fucking up (in ways different than your brother.) If you asked your brother, despite him being guilty of plenty, he might say he feels like the family punching bag. Not because he's treated poorly, but because all the attention and criticism is on him.
Despite being "that guy" to my family, I"ve made it into my 30s successfully, and my family still loves me. At some point, you either accept someone's life choices, whatever they are, and learn how to deal with them or you don't. Whether that's a bad attitude, smoking and drinking, a wife you hate, laziness, thoughtlessness, sexual orientation, whatever. The only thing you should be trying to figure out is your own emotional response to it, one you can live with that isn't perpetual angst on his behalf.
From my own perspective, being badgering and fretted and worried over made me feel guilty plenty, but here's what it didn't do: it didn't make me change. In fact, it made me go "Fuck it then, I'll be "wrong" and see how that works for me." My brother and I have always enjoyed a pretty honest relationship with each other. When we're like "Hey it's time for straight talk" it happens. But in those days even that became very strained because people simply would not stop pressing me, even when I was like "Yes, message received, thank you, enough."
I think personally you're getting uptight about a few too many things. People will smoke cigarettes, that's just a fact of life. Maybe they'll quit, maybe they won't, everyone dies one day so is it really worth harping on him now about it? I guess if you're going to go on a crusade against smoking because you believe you have the power to make him change, the answer is yes. I too went through this with my family and eventually, I won out. I get the right to make life choices without having my entire family in my face about it.
Now the binge drinking present a more immediate and life threatening problem that warrants genuine concern. The laziness and lack of helping your mother is something you can genuinely talk to him about. The smoking and drug experimentation......I think you need to let go of. Unless he's blowing his saving on coke, doing meth or shooting heroin and/or bath salts in his veins, I think he'll be ok* Most people eventually grow out of that phase.
His maybe-fiance.....I wouldn't go near that with a 10 foot poll other than to remind him the long-term consequences of his choice and get him to think honestly about her and their relationship. You not liking her, frankly, is not honestly his problem. If he likes her, tough. She sounds like a balancing force in his life, something you should probably be thankful for despite her possessiveness.
You're judging the shit out of him and considering him a failing human being for some stuff that honestly....I do not believe is a big deal. Then again, like I said. I'm the black sheep in my family.
Sorry if that reads pretty harsh, but minus a couple things, I hear a lot of echoes of my teens and 20s in this. And I came through it ok, and in some cases, completely flipped the script on people. I don't say it to my family but I get immense satisfaction sometimes from that knowledge.