I made those adjustments, reflashed everything with the latest firmware from the repository, and voila! I now have 5 fully working clocks!
I'm sooooo happy!
Interestingly, the order of factory flashed chips also arrived today. On one clock, the combination of my newly flashed ESP8266 and the factory 3.01 chip resulted in the clock not found! error. Replacing the Atmega with a freshly flashed one fixed the problem.
Thank you for the follow-up! I'm happy to hear everything is working now. A bit reiieved as well because I was pretty much out of ideas.
Libraries... always fun in Arduino-land. So many people writing libraries, no real quality control, third parties can and do introduce breaking changes without notice... fun ensues. Ian does distribute the right library versions with the firmware source, BUT if you already have the latest versions of those libraries installed in the IDE (quite understandable) things get weird. Arduino IDE has some mechanism, not documented anywhere that I know of, to make choices when multiple versions of the same library are found. I guess(?) it prefers the newest version it can find which in this case is obviously wrong. Personally I'd rather it would complain and make make me correct the ambiguity but I don't control IDE development.
Anyway, I'll remember this and next time something similar comes up I'll ask about library versions early in the exchange!
Look into it later when the dust is clearing off the crater.