Thanks Sterophonick. That is an outstanding reply. Hopefully I understand some of the basic functions of the cart a little better now.
My problem, basically, is that I'm an idiot. I thought that checking the "Backup Disabled" box meant dis-enabling backups, but it was just displaying the "Backup" status as "Disabled". I guess it reminded me of Group policy items like the one for Onedrive where you enable the rule to "prevent the use of...". So yeah. It works now that I've checked it and it says "Enabled". Thanks for clarifying. Though, if it says "Save Backup" then the "Save" part is offscreen. Amounts to the same. Just FYI just in case.
So next time, in order to recover from a corrupted save, now that save states are being backed up to BACKUP/SAVER, it seems as easy as copying the latest
uncorrupted save from BACKUP/SAVER to SYSTEM/SAVER, removing the appended number from the end, and it should load as latest quicksave after booting the rom with Boot with Addon enabled.
I'll try that out later with another game. Now that I've played back through the hours lost in that ROM I'd rather fiddle with a different one
Again, thanks.