Sent support an email, and got a reply which - if nothing else - demonstrated how to properly do a factory reset: the browser-based Meshtastic Flasher has a bin icon next to “Flash”, and if you tap that you get a U2F that wipes the microcontroller.
Throw that on the card, let it reboot, then reinstall the firmware.
Sadly, I tried that - with their recommendation of using the 2.4.2 beta firmware instead of the factory firmware - and nothing. Still dead. Sad times.