Hello Jan, and many thanks for your help. I hadn't tried Safe Mode but have now done so and attempting to install a driver again, with the same lack of result. Something strange is happening which I think you were aware of when you suggested that installed files were not being detected. This seems to be confirmed by the fact that at the end of each install attempt I get the usual green tick and "Installation Successful", but no sign of installed drivers anywhere.
Several thoughts occur to me: (i) Perhaps this pesky Hybrid Drive used in this machine has become corrupted - instead of the apparently intended Sys and Data partitions displayed in place of Macintosh HD, I have Sys, Data and Sys-Data (which I have had to rename Sys-Data2 to allow Time Machine backups - apparently a known issue); (ii) Maybe I should create a Catalina usb installer and do a new install of the OS and see how that might affect the drive situation, as well as printer installation; (iii) Restore from a recent Time Machine backup, which will at least return the original partial printer function (not potentially helpful I suppose).
Alas my Mac Mini has died, so using it as a print server is not an option. I have a 13" MBP running 10.13 with full printing and scanning function which could be used in the same way though.
I'd appreciate your thoughts on my clutching-at-straws ideas above, and thank you again for your valued help.