I now have some diagnoses and codes. Their comments are given as quotes. My comments in italics.
i.e. the 12 V battery was drawn down considerably, but not faulty.
Batteries reading 3v, charge battery and test with rotunda tester, indicates good battery.
this is rather startling, given that the car had "stop safely now" either two or three times, a "low 12 V battery" warning once, that was sent to the cell phone, and two hard shut downs--all apparently without successfully storing a code onboard the car!
Connected 220 charge cable, Charged lithium battery. Run self test on all modules, no stored codes
Not nice that it happened to them, but thank goodness, because it gave them something to diagnose.
Road test vehicle, vehicle stopped, and stop safely now light is now on.
whatever PPT H is, I didn't ask
Performed self test on all modules, PCM has a stored code D0A0AI01 High Voltage System Interlock Circuit. Perform PPT H
While not listed in the diagnosis, they also replaced the powertrain control module (PCM) as well as finding and fixing loose connectors. Unclear to me if that's because the pins are integral with the PCM, or if there was some additional problem with the PCM, or if they mostly did the replacement out of an abundance of caution.
Remove left from wheel, inner fender panel to gain access to PCM, remove PCM and connector C175B. Perform PPT H1TO. Performed wiggle test on all connectors check pin fit on all connectors C1822B, C1822E, C1822C, C1822A, C1822G, C175B and C1457A. P1 on C1822E and Pin 2 on C1822G Pin fit is loose, repair pin fit and reassemble vehicle. Run on demand self test and all passes.
We're having a significantly worse than usual crop of potholes and other assorted road lumps and debris this year, and the car has had more than its fair share of sudden drops and bumps in its short life. So connectors could have been loose from the factory, or could have been jostled loose.
FWIW (not much?) the Left front wheel was also the wheel that they had replaced a tire on a couple of days previously (my bad--slashed it on a rock). I'd assume that's unrelated / incidental, but for completeness, I mention it.
I don't have a tester to confirm the state of the 12 V battery, but I'm being far more judicious in using it--especially when the car is not a) in drive and ideally b) in motion--and have not had more problems of this nature.
I still have other (ongoing) minor weirdness with the navigation (which likes to go to a blank screen, while repeatedly demanding that I "pick a line") and the entertainment (which still sporadically tells me that my files are corrupt or unsupported, and then, often as not, plays them anyway). But those are quirks I can live with.
I am pleased to report that as the weather warms into the mid 50's, the car is now allotting me 102 miles worth of driving for a full battery--much better than I'd hoped for--and it continues to drive very enjoyably. I'm still glad I have the car.
I should also mention that my last car was one of the very first US Priuses (delivery in Nov 2000). That Prius stranded me a couple of times with its own version of "stop immediately" until Toyota got things figured out, and sent out an operating system patch. (And that was AFTER the Prius had been out in Japan for two years.) If Ford is as diligent about their teething problems as Toyota, I look forward to a mostly drama-free 8 or 9 years...starting (I hope) very soon.