Occasionally no power to start

You should be good to go now. Too bad you were misled about the problem.

Technology improves, but 12-Volt batteries still only last 4-6 years, max.

 

Perfect. That's what I'd do and that's the conclusion I'd have come to as well. Getting the same voltage both ways confirmed that a loose connection was not keeping the battery from giving all it had to the car.

 

If they said yes then they’d be on the hook to owe you a replacement or prorate battery. It was smart on their end to say no and not deal with it. Not surprised.

 

2012 Plug-In with panel entirely dark, and both batteries fully charged. What are my first DIY steps?

 

look for the fuse

 

Fuse, a loose terminal, or a bad ground connection most likely. I'd check the connections first.

 

Perhaps this issue deserves a new threat. Here is where I am so far: smart charger directly on the battery terminals shows it fully charged, but panel is dark. We know the main battery is fully charged from a few days ago on the home garage charger, driven just a mile or two since.

 

Thanks guys. Where would I find the relevant fuse?

 

Gather the low hanging fruit first.

I don't remember if there's a fuse at the 12V battery, but there's also one in the fuse box under the hood if I remember right. But the connections only take a second to check and they are more likely to be loose unless you've been doing stuff you haven't told us about.