The Story Behind Our Personal Crisis
The easiest way to determine if your battery is ending its time is to follow your initial practice. Then, you know for sure that it is Dead. Just sayin'...sorry that you had to learn the hard way this time. More importantly, thanks for posting your experience. When we buy our EV, I will keep your experience and suggestions in mind.
Good advice and I'm about to make a video on this topic as well. One thing I'm surprised about is that your car shut off when you removed the external 12V power source. The DC to DC converter (the thing that acts like an alternator by converting 400V power from the traction battery to 12V) should have been actively charging the 12V and thereby keeping the car running. So you should be able to "jump start" the car and then drive it until you find a place to swap the 12V. Obviously that wasn't the case for you, so I find that curious.
Anyway, the advice to change the battery before you need it is sound. Here in Michigan, the 12V batteries tend to die in winter then the electrochemical processes don't work as well in cold weather. So this is the time to check and replace the battery up here in the frozen north.