Shouldn't have asked for 'database experts' to start with, that moved things to a wrong direction...
All while the solution was THAT simple -- once found out, it only took 3 steps:
1.) In a column view, filtered to include all files with actors (e.g. by "-[Actors]=[]"), add your [Actors] column, or whatever called in your language, and sort it by 'Number of files' (or click column header > Edit > Sorting).
2.) Next, scroll the column down to the first actor who has only one film displayed in the file panel below; then continue selecting all entries further down ('Shift + End', or 'Page/Arrow Down' to select individually, Ctrl+Click works too).
3.) Now press 'F2' on your keyboard to open the first selected entry for renaming, then press 'Delete' or 'Backspace'. This will transfer the new 'empty' value to all items in your selection, thus clear them in this case (quite like renaming in Windows Explorer really ;-).
Et voila, all one-film-only actors have been removed from your library! (In my case of ~10,000 actors, it took a few moments of background activity to complete... ;-) Note: Should you wish to keep a complete list for backup, create a new field, e.g. [Actors (complete)] or whatever, and copy them all in there before doing the steps above.
I wonder why nobody here, however helpful and interesting some comments were, had thought of this simplest way... Perhaps we're all too obsessed with more new stuff, new versions, new technologies etc to outwit the others to remember the very basics...
(BTW, for entertainment's sake, just been watching 'Rogue Cop' (1954) while writing this -- good old dressman, Robert Taylor is sure to stay in my library, most other crooks are not! ;-)
(Edit: topic subject changed accordingly)