1) DO NOT try to install a different model's firmware (even if you could figure out how to do it. The official image packages are model-locked for a reason).
2) To get fsck to run, just connect the Kindle to a wall charger and re-boot it.
The boot-up routine will test, and correct as required, any file system errors.
BIG NOTE: fschk'ing eMMC takes a long time (if there are errors), so you must leave it on the wall charger until it completes its boot-up (which may take a week or longer).
STOP at this point and do not proceed until a week to ten days has past.
3) DO NOT use the "Erase All" command found in some of the various start-up stages.
It does what it says, it does work, you will be very, very sorry if you try it out.
4) You can get the full system image for the 'main' operating system from Amazon:
https://www.amazon.com/gp/help/custo...deId=201630670
Alter the version number in the download link found on that page for any other firmware version.
= = = =
If the above is ALL followed, and the device is not 'fixed', then enable the 'capture' feature of the terminal emulator you are using on the serial port, re-boot, and when it finishes going as far as it will go, post the resulting 'capture log file' as a compressed attachment to your next post (in a week or two).