You have raised many points but did you mention anti-rollback?
As for a preinstall checklist for following readers, I propose
Take a screenshot from the FairphoneOS showing the Android Security Patch Level at the time of unlocking.
This will seem an odd suggestion if one has not noticed the Red coloured Caution on the install page
Please read that caution. This post discusses the issue (answered) Install /e/ on FP5 with newer Security Patches