Kindle Paperwhite 5th Gen (2012) Jailbreak Removed but firmware update still fail

I have a Kindle Paperwhite 5th Generation (2012) running firmware 5.3.3. It was jailbroken and works normally when powered on — I can open KUAL and use MRPI — but anything related to updates or rebooting is broken. The "Update Your Kindle" option is available and runs without errors, but after the Kindle reboots nothing changes and the update file is still present on the device. I tried using MRPI with the jailbreak uninstall .bin, but MRPI reports "installation failed." The "Reset Device" option in Settings does work and it removes the jailbreak (KUAL no longer works afterward), but even after a factory reset I still cannot install a new keystore file or update to a newer firmware — the update appears to install, but after reboot the device stays on 5.3.3. I also cannot access diagnostics mode (ENABLE_DIAGS doesn’t work). What is the safest way to completely restore or reflash a clean stock firmware on this device so that updates will work normally again?