What's the quickest way to get the app to take my external edits to columns.ini? Not F5, I find surprisingly.
Restart the program
Unless you're one of those people who launches Mp3tag with their entire 150,000 file library loaded. In which case I'd say you picked absolutely the wrong time to be manually editing columns.ini.
Although possible for people who understand what they do, external edit of Mp3tag system ini files is not recommended. Sometimes it includes the lost of automatical update the changed values.
To get external changes of the columns ini file into the running instance of Mp3tag, a possible way to go may be via the Column Setup's Utils dialog.