Sooo, I think I've bricked my PW4 for good:
As I hate Kindle Feedback item in my library I've removed its creation from /var/local/mkk/bridge.sh.
Everything went fine, except PRE_GM_<blah> was still created every reboot.
That's when I've found /var/local/system/fixup script.
I've again modified it to skip PRE_GM flag setting, but I've made a typo somewhere ;(
Now the dearest device is in a bootloop, without any chance to even inject emergency.sh...
EDIT:
Fixed!
Being away from my workshop I had only essential toolbox with me. Unfortunately no UART converters there, but I found some stray Pi Zero, hooked it to Kindle's HW serial, entered kernel recovery mode...
There I was able to mount fat partition and upload emergency.sh with some recoveries.
I'm up and running again
