More background from MadMAXXX's debricking-via-fastboot thread:
https://www.mobileread.com/forums/sho...=269099&page=5
Implies that for whatever reason and whatever motivations, the diags system just isn't working anymore.
So hopefully if he can actually get his hands on the elusive diags image, he can fix what he broke, and then finally have the opportunity to repair the main partition via Amazon's supplied rootfs.img