Solved! …mostly. I decided to go through all the steps one more time, taking note of everything I had learned this far. In that process I thought of the batteries. I had actually thought of them before, but discounted that possibility because of all the claims of low sleep power consumption, even with the radios on. Since I had always set the meter to shipping/no radio when putting it away and had made so very little use of it, there was no chance of dead batteries. Also, the processor was running and was able to reflash the firmware. Let’s check anyway…

…1.0 volts on each of them!

New batteries and it’s working again. But, the question of why they were dead remains. They were the original batteries it was shipped with.

New problem: I selected manual voltage range, it went to 600v range and the app crashed. It took me a while to figure out that that state was in the meter. After a battery pull, I got it going again. The app crash was 100% consistent on both my Marshmallow Nexus 5 and my Lollipop Nexus 9.