Question: Startup Security Utility won't Unlock
This is a followup question and answer to another post at iMac Pro startup woes. These apple discussions won't let me revive the old thread, so hopefully this makes the google cache and helps some others.
Basically any attempts to turn on the firmware password through macOS System Restore > Utilities > Startup Security utility failed at the password prompt for the administrator with a message of "Recovery is trying to change system settings" enter password. I found a solution and will post it in a reply here.