Documentation is always great for court. I think there should be a letter like your lawyer suggests. I'd make a few changes, though, namely...
(1) maybe instead of stating "sober for six months or I'm filing for divorce" to keep it a bit more wishy-washy (a.k.a. "reevaluate our relationship in 6 months" or something to that effect, as if you decide to a divorce anyway despite any of her efforts it doesn't look like you not holding up your end of the agreement
(2) be specific about your concerns especially with regard to how they affect your child but don't be specific about the resolution you're looking for. You don't want to give her a checklist to follow, you want to make her think about what she needs to do to resolve the issues you present, as her thought process in this will show you and the court whether she is truly trying or simply hoping to dodge her rightful consequences. If she has a checklist to meet, you can't show her motives as easily. (Of course I'd spell out the obvious, like not drinking when she's the only adult in the house watching your child, or no drinking and driving with your child if she has a habit of that, but leave the other things in the gray and let her figure out a plan).