Yes, the downgrade involved a factory reset.
I have been working the last hour or so on trying it all a different way. I flashed the last version of A12 with the Android Flash Tool, and then tried the /e/ Easy Installer. Same result, essentially. When I was instructed to select Recovery Mode and confirm it with the power button, the phone then looped a couple times, showing the G-logo then back to the fastboot mode screen. The phone will not boot to stock or recovery now.
What would fastboot -w do for me?
At this point, I think I have to reflash the stock ROM … again.
These issues seem very much the same as those discussed back in February in this thread: https://community.e.foundation/t/google-pixel-4a5g-stuck-at-fastboot-will-not-switch-to-recovery/47230
I just had hoped that I could avoid the solution there, which is to go back to A11 …