Re: "more of a super-cruise-control"
Great - my car can't do that, it can hold lane and maintain speed and distance to the vehicle in front.
I would suggest that any system that is sufficiently good to lull an idiot (I'm not suggesting you are, I think you're arguing a point and not suggesting an action) to try seeing what happens if they climb into another seat while driving, should be allowed unless it is capable of safely stopping. The problem I see with systems such as Tesla's Autopilot is that for a minority of owners they will assume it can do things it was never designed to cope with. The problem isn't so much a technical issue as one of human understanding. If it seems to be able to drive under certain circumstances, some owners will assume it can cope with more.
I might have lifted my backside off the seat to adjust position
In which case you'd only lifting yourself off the seat squab for a very brief time, too long and the car should be reminding you of your responsibilities, it sounds like it is a little over eager in this respect.
If, as Elon has tweeted, AP wasn't even installed....
Then either driver left the seat after the accident, we'll have to wait for accident investigators report, but they'll probably know this. Or the driver was being even dafter by trying to control the car from the passenger seat.