Nice to have met Honeybug. You have some great pictures in that album - looks like a wonderful trip.
Your car clearly did stop with charge left; but perhaps lack of charge was not why it stopped? It could have been some problem that made it take the car out of gear and keep you from putting it in gear. That would explain why you didn't get any messages about the car shutting down, as well as why it couldn't be reproduced by low charge.
Frustrating either way, I know - the car still wouldn't go when it should have. But it doesn't necessarily mean that we have to start worrying about the accuracy of the rated miles display.
By the way, when my car was fairly new I once lost power going up a hill - very disconcerting. It turns out I was going for the windshield wipers that are on the other side in my other car, and brushed the gear selector and put it in neutral. I know that's not your problem though, as in my case it went right back in to gear when I asked it to.
So what would keep the car from going in to gear? That doesn't leave any errors on the screens or in the logs? And doesn't get fixed after a reboot, but does fix itself after some time and a charge? This smells like a flaky sensor issue, but it seems most sensors giving readings bad enough to keep the car from going in to gear should result in a message of some kind.
All of which prompts me to wonder: since the Model S has only one gear (and isn't it the same gear for Drive and Reverse?), what actually happens when you 'put it in gear'? Is 'freewheeling' in Neutral just a software fake job so that the motor provides no discernible torque or braking, or does it actually disconnect the motor from the rest of the drivetrain?