The MrPI scripting is not intended to install Amazon packages.
Series 4.x firmware still uses incremental updates, that is not a supported 'feature' of MrPI.
You should be able to replace the certificates, but try this first:
https://www.amazon.com/gp/help/custo...deId=201994710
Note that installing that services update will not change the firmware version displayed by the Kindle.
So there is no easy way to tell if it has already been installed.