My PC is officially unclogged. The extraneous files have been obliterated, the old install and last traces of the old install are no longer part of the map.
...though the old install still turns up on the startup and I don't know why...
Like all desperate bargains, I paid a steep price for the fix - at least half my Firefox tabs and windows.
I can rebuild. I've done that before.
I got a couple of hours before Beloved and I go out for lunch and I drop her off for the usual trip to Adorable. This week's got Beloved and Adorable wrangling A Thing so the rest of my week is a little sparse for company.
Let's get on with all of the usual.