-- ISSUE RESOLVED. --
Though, I failed to use fastboot on this specific Kindle - it also seems to have a different U-Boot version btw (2014.04).
I managed to get around this by typing
run testboot in the U-Boot prompt - this command would make the system boot from the MAIN rootfs regardless of your current bootmode parameter, since all it does is to run the script defined in the 'testboot' environment variable, which is exactly the MAIN boot script.
After this, you can get to a terminal and run idme to fix your bootmode arg.
Lesson: check your DIAGS rootfs before trying to even boot from it - Amazon has apparently ripped that from the system