it's unclear if I had two separate problems, or one problem that magnified. But one thing that clearly was wrong, in conjunction with the "stop safely now" warning, was a drained 12V battery.
Now, it's possible that the hard shut down I described helped to drain the battery. But it seems likely that the battery was already low.
The dealership reminded me that the 12 volt battery does not charge off of the large battery. It does not charge off of the plug. It does not ever charge, in any way, except off of the wheels, during regenerative braking. If you do a lot of coasting (rather than accelerating and braking) you may starve your battery, depending how high your use is. (I was aware--had been told--that it didn't charge when the car was off, but I didn't remember quite how limited the charging was.)
Things that drain the 12V battery: pretty much anything electrical that does not show a "hit" on your mileage for the big battery. I believe this includes
* lights, including flashers and cabin lights
* sound system (like listening to the tail end of a radio program or song after parking)
* voice interaction
* checking in via your phone app, which apparently wakes the car up for 20 min or half an hour each time (!) [This is a bad design choice--especially if they are going to promote leaderboards, showing the interface to friends, etc.]
* possibly, those annoying moments when they car tells you, yet again, that it's cold, even if you have not checked in with it (and even if there's no way to plug it in, to make it stop complaining).
* start button
In theory, I see no reason that the car could not keep driving with a dead 12 V battery, and a fully charged big battery. But it seems like that's not actually the case. In theory, it strikes me that, seeing they have a "low 12 V battery" warning, it should come on long before the battery level interferes with driving. But maybe that's not the case. In theory, it seems to me that they should be able to have a connection in the car where you can press a button and charge the 12 V battery from the big battery (but this does not exist). In theory, several of the things that run off of the 12V battery should probably be reconfigured to run off of the big battery (either by default, or optionally). In theory, if 12V depletion is a big problem, they could have a 12V charge port/plug and sell the car with a 12 V plug in converter/cable.
Anyway, my current working hypothesis is that my "Stop Safely Now" problems are linked to draining the 12 V battery, and that good 12 V battery "hygiene" should be able to avert at least one major cause of "stop safely now" warnings.
I was also told that "some clips were loose, and some other stuff." They were prepping the full list of what was done (diagnosis and repairs) for Ford, and will mail me a copy, at which point I should know more, and will post another update. I have to say, my local folks were very good about this, though it was totally new to them (and therefore took a while). So, a shout-out to Day Ford (if I'm allowed to do that.)
I don't know if "Stop Safely Now" causes downstream faults that can only be rectified at the dealer, or if a plain old jump to the plain old 12V battery might fix the problem. (I'm not too keen to be the guinea pig on that, unless Ford OK's it... but if anyone has tried it, please share the experience.) Having had several cars from the the 1960's (the generator era, where you also had to drive to recharge) I know that if a 12 V spends much time drained (or if your generator is bad) the battery doesn't hold charge very well when it is jumped (that is, a standard 12V car battery really isn't a rechargeable battery).
Anyway, if you are worried about "Stop Safely Now," keep your 12 V battery happy (it can't hurt) and have pretty much anyone check the health and strength of your 12 V battery if you're been drawing hard on it.