Honestly...what makes you any better than your ex and his wife? Yeah I know, you've pointed out your list of "virtues"...you don't smoke, you don't do drugs, you only drink occasionaly, you're a foster parent and you refuse to "sling mud"...oh yeah, you're a regular Mother Theresa. On the other hand...while you're putting down the ex and his wife for their recreational drug use and their "convict" friends and telling us how bad their alcohol use is...excessively 4 times a week AND to the point where the wife blacks out a couple of times a week...it can't be THAT bad if 1) you've done with 50/50 in the past and 2) are still willing to go with 50/50 for the future.
So what is the REAL story here? Is your shiney, virginal pedestal really more important than your child? Is it the "control" of having "custody"? Or is it the fact that you fucked up and can't use the drugs and alcohol and are looking to us to bail you out...WITHOUT tarnishing your "perfect" image?
Personally...I would NOT have a problem with having my child grow up and finding out that I was willing to roll around in the gutter if it meant keeping him safe, rather than him growing up and finding out that my "image" was more important than his safety.