My Take On Things
Okay here is the way I see it. I have seen this fascinating documentary about 7 times now.
What was wrong with Diane on that day seems clear to me. Migraine. I'm not sure but I think that would not have shown up on an autopsy.
Jay saying that Bryan said "mommy's head hurts and she can't see" that is classic migraine. It can cause vision disturbances and pain ranging from mild to excruciating. So that explains, for me, what happened to Diane that day. Though I still believe Diane & Danny had a terrible fight, life-changing, at the campground the night before, but we will never know if that is true.
Ok, that sets up what happened, in part 1. Part 2 is What Diane Did About It. That is where Diane's mish mash of a mental state comes into play. Any normal reasonable person in the same situation would have stayed in the McDonalds, or pulled into a rest stop, told the kids, I can't drive us home, I can't see.
Then since Diane couldn't have called her husband, she could have called her brother, and told him to come and get them.
But that wasn't what Diane did. She pushed through, because she always pushed through, the migraine's effects got worse and worse, and we know the rest.
Gee Woodle, Space Kadoodle!