Thanks so much for replying!
I actually already tried what you outlined there, but it didn't work... Well, the downgrade itself worked, but when I try to install the hotfix the last time, Kindle gives me an (official) error screen: "Update failed. Sorry, something went wrong. Press the power button to restart your Kindle."
I tried restarting twice anyway in case it actually worked, but nope... I also tried installing the hotfix by dropping it in the mrpackages folder and running ;log mrpi... It initiates, but then gives a message saying something like "not targeting your FW version, skipping" and the next message is "Destroying package . . ." It then loads a completely blank (but backlit!) screen, and I have to hold the power button to restart...
Edited to add the correct MRPI error message. I thought this was supposed to work with 5.16.2.1.1......?