Thanks. Really appreciate you taking the time to respond and all the work you do on LE.
For now I've solved the problem by putting LE to the side and installing Kodi on Raspbian instead. Raspian allowed me to pre-configure the WiFi, which meant I could enable the webservice and then use the iOS remote app to navigate the Kodi setup process.
For what it's worth, I still couldn't get CEC to work. The same TV/AVR/remote works perfectly with other LE boxes, so I suspect you are correct and something weird is going on with the mini-HDMI cable (this is the only mini-HDMI device I own).
I would have spent longer fixing the cable in order to use LE, but really the was just an experiment to see how well Kodi would run on the Zero W (the answer? Surprisingly well). This box is ultimately going to become a Home Assistant server.