Great concept, and consistent with everything I've read before regarding story structure. The protagonist has to face that question, determine whether they are or are not ready to risk everything for something or someone, and choose in the affirmative.
I just turned in the novel that is my MFA thesis project, and I had no trouble identifying its mirror moment. No mirror was involved for my protagonist, only the recognition that he'd just gotten the last piece of information necessary to prove his former friends were guilty of an evil he couldn't look away from, even though taking action would put him on the wrong side of some dangerous people. That moment was the midpoint of the book, and his decision propelled the action of the second half.
Mr. Bell's book sounds like one that needs to be on my shelf. I love good books on craft. Thanks for reviewing this one.