UPDATE: (3 Oct)

Incredibly, I can dare to hope again!

@deathwing in Discord told me not to worry and pointed me to HiveWorld where I was able to use my Old Master password to obtain my Old Owner password as well as all the public keys from the Old configuration. He also pointed me to the Account Recovery tool and helped me understand the process.

I returned to the excellent document on recovering an account by @abitcoinskeptic. This time I read it properly, not distracted by my state of mind. This post gave me an in-depth explanation of the process.

I'm very grateful to the two of you, as well as @asgarth who pointed me in the direction of the Hive server. I now believe this is actually possible and that I will do it!

The Current Updated Position: I have the keys required to initiate an account recovery. I pretty much understand the process. I have a short 9 minute window in which to request and initiate account recovery. This window will appear 30 days after 1 Oct (pretty sure that's the 31st).

The window will run from 30 days after the time I changed the recovery account for @barge until 30 days after the time I last changed the @barge account keys. Time is in UTC and clocks go back around end Oct, so I'll need to be super alert!