The MC Critical Flaw is the quality that undermines the MC's Unique Ability. In Success stories, the MC Unique Ability is more effective than the MC Critical Flaw. In Failure stories, the MC Critical Flaw is more effective than the MC Unique Ability and therefore scuttles the MC's ability to help achieve the Story Goal.
The MC holds the key to achieving the Story Goal, but does not have to be the one to use it. You can easily imagine Dr, Watson (often the MC and almost ALWAYS the MC in the novels) making a casual observation or discovery which Sherlock Holmes (the protagonist) seizes upon and uses to solve the mystery. Since MCs are often Protagonists, it seems as though the MC Unique Ability is tied to the Protagonist. This is not the case and only appears to be connected because of the MC/Protagonist storytelling convention.