Ok, I was able to fix things myself -- KUAL and KOReader are opening and working fine now.
The key was that I needed to reinstall MRPI -- this was why I was getting errors during MRPI install about "targeting your FW version." I thought it was a problem with my Kindle's FW, but it was a problem with MRPI.
Once I updated MRPI w/ the files in NiLuJe's snapshots post, I was able to install the JB hotfix through ;log mrpi with no issue. Then I installed the KUAL (coplate) hotfix through ;log mrpi. I can now launch KUAL no problem, and was able to run "update OTA bin" through KUAL. Finally, I installed the hotfix for KUAL Booklet (to quick-launch KOReader), and all is working perfectly.
tl;dr, this was what worked:
- Accidentally update to 5.16.6.
- Follow instructions here re: installing the 4 "emergency" files.
- Drop the official .bin for 5.16.2.1 or 5.16.2.1.1 into the main Kindle folder; eject/unplug Kindle; update via UYK.
- Update MR Package Installer (from snapshots page), then restart (i.e., replace your Kindle's current extensions/MRInstaller folder with the extensions/MRInstaller folder from the snapshots page; DON'T DELETE THE OTHER ITEMS IN THE /extensions FOLDER!!)
- Install NiLuJe's K5 JailBreak Hotfix via ;log mrpi (hotfix file from snapshots page), then restart twice
- Install KUAL hotfix via ;log mrpi
- Install KUAL Booklet hotfix via ;log mrpi