I want to dispute the Deus ex Machina example. In my eyes, the example fails to meet criterion #4: "Deus ex Machina are external to the characters and their choices throughout the story." The students are the protagonists of the chapter, and one of them is choosing to use this mirror in a clever way. The example seems to be arguing that the presence of the mirror is somehow "external", but I find this unconvincing. A mirror is a common household item; it is not implausible that at least one student out of an entire class would have a mirror in their desk. The mere absence of Chekhov's Gun is not automatically a Deus ex Machina. Now If the student pulled out a magical amulet that reflected spells back onto their owner, I would call that a Deus ex Machina, since there would no plausible reason for the student to have such an object outside of the need to defeat the chapter's villain.
I want to dispute the Deus ex Machina example. In my eyes, the example fails to meet criterion #4: "Deus ex Machina are external to the characters and their choices throughout the story." The students are the protagonists of the chapter, and one of them is choosing to use this mirror in a clever way. The example seems to be arguing that the presence of the mirror is somehow "external", but I find this unconvincing. A mirror is a common household item; it is not implausible that at least one student out of an entire class would have a mirror in their desk. The mere absence of Chekhov's Gun is not automatically a Deus ex Machina. Now If the student pulled out a magical amulet that reflected spells back onto their owner, I would call that a Deus ex Machina, since there would no plausible reason for the student to have such an object outside of the need to defeat the chapter's villain.