So... I forgot the password to this site and eventually decided to abandon self-hosted Ghost
When a gem updates itself and breaks a lot of stuff, it's easier to go back to an earlier version than fight to solve all the issues at once.
Although this is the lesson I took from a recent experience where a gem updated and broke the main functionality of the site, I probably should learn the value of keeping my local bundle updated. That's a harder lesson to learn. ;-).