This has largely been solved by @Geoff_Coupe here, thank you.
I would like to suggest as a feature request that a clearer way be introduced of distinguishing between Genres in Roon’s metadata database and those in my own hierarchy - as picked up from the appropriate tag in my own files: it’s currently effectively impossible to know which (of these two sets) is being edited in the Album Editor’s ‘EDIT ALBUM’ > Genres dialogue.
Thanks!