...I imagine that "showing that the update has not broken configurations or dependencies required to boot" would be addressed by having testing the configuratio

...I imagine that "showing that the update has not broken configurations or dependencies required to boot" would be addressed by having testing the configuration on an exact mirror of the production server, including rebooting, right? (of course, after a reboot the testing server is technically no longer a mirror of the production one...) ...of course, the OP should enlighten us about whether this is how they did things