TLDR if you are on PW2 5.12.2.2, install KUAL by command
in search bar.
if you are on PW2 5.12.2.2 like me, after
Jailbreaking, the KUAL-KDK-2.0.azw2 is still not working and giving this error "This device is not authorized as a test Kindle by the developer of this item".
Turns out that I can install .bin by the MRPI although the "install update" is greyed out on 5.12.2.2.
put Update_KUALBooklet_v2.7.37_install.bin under the /mrpackages folder, and then run
in search bar.
It resolved the issue for me.