I guess, currently ftinspect requests user to manipulate GUI to choose the fonts to be inspected. It is convenient for the users who are unfamiliar with CLI & hard to understand how to choose the fonts. But, if possible, I hope if ftinspect can accept the CLI arguments to list the fonts to be inspected. If the fonts are located various directories, or, some hidden directories (like ~/.fonts for fontconfig), choosing via GUI is slightly stressful operation. Here is a patch to check the file readablities in the argv list, and if it is readable, pass them to openFonts().
But, I'm afraid the introduction of a public method MainGUI::loadFontsFromArgs() looks quite irregular in your design. How do you think about?