I know people (friends and family) who drink and am familiar with their habits. Some people drink a lot, some always just have one or two, some are seasonal (I'm more likely to have a drink in the summer), some are in AA and don't drink at all. There are obvious differences between these people's habits.
The docu doesn't show anyone who ever saw Diane impaired, drunk, drinking more than one or two. Yes, she could have as she was home alone every night. But, she had never been in an accident before, no one saw her driving impaired before, she had a professional job, took care of the house and kids. Even if she was a functioning alcoholic SOMETHING went severely wrong that day. She was driving aggressively and somewhat erratically for many miles, stopping and looking like she was vomiting after/around the Tappen Zee Bridge although she had seemed fairly normal at the McDonalds and the gas station (although I found her driving there aggressive). But by the time she enters the Taconic the wrong way (after first going the right way and circling back) she looks "determined" she never wavers once she gets in the passing, fast lane going the wrong way-she had to cross all lanes to get into that passing lane. It was noted there was no lane shifting, no movement of the car, she held on to the wheel, stared straight ahead and never looked at or acknowledged any of the many people who were flashing lights, honking horns, moving quickly out of her way on the ramp and on the highway. Not being an alcoholic I don't know whether this behavior is consistent with alcoholism but I can't imagine me being able to do that drunk or sober as if I were drunk I wouldn't be coordinated enough to drive that straight and, if sober, I wouldn't be crazy enough to drive into oncoming traffic without wavering.
Before watching the docu (and I haven't followed this closely but was aware of it) I was feeling like, "OK, she was a functioning alcoholic, hid it well, drank too much and had an accident" simple tale. I'm not sure that's what happened. I understand her husband's reaction although I firmly believe it would have been easy to hide ANYTHING from that guy. He was and prefers to be, clueless, and depends on others for everything.
But there were many other people in her life who knew she smoked pot, who spent time with her and who seemed to know her pretty well. It was noted that she never discussed personal things with anyone. Her marriage, feelings stuff seemed off limits.
I think she created an image of who she wanted to be and had been successfully presenting that image to the world. She had been "abandoned" by her mother and refused to re-connect with her since the age of 9. She was a perfectionist married to a..kind of lazy, underemployed slob-even his mother kept repeating he was just like Diane's "oldest child".
Did some combination of things happen to make her snap? What happens to make a wonderful mother, responsible person drink and smoke pot while driving 5 children in her car? What the heck is the 9 hr phone call story and is it true? Did she just accidently drink too much and panic when she realized it and try to get away from her brother and avoid having someone find her out and just had a horrible accident? I guess maybe..
I DVR'd the next time the docu shows on Saturday so I can watch from the beginning but I think I'm more mystified than ever.
Thanks for the research, tamfish.
In answer to your question, I think there are people who drink, but never do so to the point of being visibly impaired.
To someone who has never seen a loved one visibly drunk, the distance between 2 daiquiris in the evening and twice-the-legal-limit drunk and driving in the morning is quite far.
But in the you-tube video, he says "diabetes", not type 1 or 2 -- just diabetes. 
