You will leave when you are ready. And to be honest, you won't "ok" knowing he's doing poorly. A lot of SR members come here daily riddled with guilt, frustration, sadness and anger, knowing that their former partners are digging their own graves with both hands. We all do that cartoonish move where one hand gingerly reaches out while the other pulls it back. That's ok.
The mere fact that you are here, questioning your situation, asking questions, struggling to find answer, is a good thing.
Also, regarding your concerns:
1. Yes Cory has had it tough, but it's not your responsibility to "save" him from his life. He has also made some CHOICES (alcohol, drugs) that had nothing to do with his past, and you are now suffering the consequences of his choices.
2. More than likely, things will get worse when you leave. This is not something you can stop, because the problems are greater than you.
3. Just because you don't feel accepted in your own family doesn't mean you need to take refuge in his. This is definitely an issue I struggled with as well and I'm now working on finding my OWN acceptance within myself, and not in others.
4. Methinks that you've already lost your best friend to his poor choices.
I can totally understand that this is a hard thing to go through, but think of this...you're still young and you're getting an opportunity from HP to learn a great deal about yourself. Think of what you'll be at 30! Watch out mama, here she comes!