Automatic Comment Notifications to Authors

I'm sorry but I didn't follow you. If commenter is not an admin nor an author he must always subscribe to get email notification. If user is an admin or author you should enable "Anyone posts a comment " of "Email me whenever" option in Dashboard > Settings > Discussion admin page.

If I have a guest author write a post, and a regular user leaves a comment on that author's post, will the guest author get an email notification that there is a new comment on the post they wrote?

i.e. is it possible that not just admins get notified of new comments, but authors do too?

Guest author is not standard WordPress function, so this can't work. It needs UserID to generate and send emails. If a post doesn't have UserID then this post doesn't have any author for WordPress.