My general take on something like this is that a cover appearance on a comic (rather than a new magazine about comics) is Creative content (whereas a cover of a magazine like Wizard or Marvel Age is Editorial or Marketing content), and therefore is generally going to count as a first appearance and be the desired book, regardless of whether the character appears in the story. But that means you're going to have another key book, where the character first appears in the story. For example, Kamala Khan's first appearance as Ms Marvel is desired both on the second print of Captain Marvel 17 and in Marvel Now Point 1. Gwenpool isn't at all related to the story in the comic that she first appeared on the cover of, and that was the hot book.