Many thanks, Jim. I tried this with just a few albums on my MacBook. Using the search in the upper right on the Standard View screen, I copied a compound diacritical (accent grave) into the query field, which pulled up several files (from Debussy) as results. I selected two to experiment on, pasting that same compound diacritical character in the "Find what" field in the RMCF pane and a single quote in the "Replace with" field. When I clicked "Save" the correct substitution occurred. I checked the relevant two media files in the MacBook finder window, and the substitution had been made there too. So am I right in thinking that the single operation affects BOTH the media files in question AND the MC library's database? If so, that's great news.
I will now test (via Splashtop on an iPad, since the Mini is headless) whether I can successfully copy and paste the square symbol into the "Find what" field. If I'm lucky, even though the same square symbol displays for both double quotes and question marks, the one I copy and paste will only identify one of those characters, and I can replace the files for each separately. If that square symbol finds both " and ?, then I'll replace them all with single quotes, since there are many fewer question marks in the affected filenames, and replacing them with a single quote won't matter to the human-readable tagging anyway, as long as the media filenames match the MC library.
If this method works in a test, I can mount all the volumes and run the search on the entire library at one time and do the edit on all the affected files in either one or two passes.
Please confirm, and thanks again.