Thank you for your input. I really appreciate it! I'll keep it all in all mind. I admit that I'm just not crazy about All Files being the default fallback solution when No Files are selected. But that's mainly a personal issue. Because I almost never want to run TagMechanic on all text files in an epub.
I'll try hard to come up with something that can accommodate all of us.
One big concern I have is having three different strategies for the three different scenarios. I would find that confusing. I'll look into creating a preference that would allow a user to opt into (or out of) an All Files default fallback (as opposed to failing completely when no files are selected in Book Browser).
Thanks again!