P.P.S The option "Automatically set author sort" in the bulk metadata dialog had the expected effect on both "Dante Alighieri" and "Jeff Garrity" and corrected the error messages in the individual "Edit Metadata" dialogs. I wonder why the "Recalculate" button in the "Manage Authors" dialog didn't work.
The only downside is that "Cervantes Saavedra, Miguel de" became "Saavedra, Miguel de Cervantes". I fixed that with the option "Edit sort ..." in the popup menu for authors in the left-hand pane. This fixed the author sort without provoking an error in the "Edit Metadata" dialog.
Checking the box "update title" as well also parsed the foreign articles (see
this thread) correctly.