Well, I'm not really sure why you think your analogy is more effective because it's simpler. The story is modeling a complex social situation involving a teen (with an unfinished brain, and a history of behavior to underestimate potential threats), and a shadowy group of authority figures. It's not a simple situation.

We as readers do not understand the specific limits of the Coven's formal authority. Neither do we as readers have any idea how much actual power they may or may not have to back up that authority. This is EoF's universe, on top of that, it's EoF's own story within that universe. What we know about the universe comes from two sources: What EoF chooses to tell us explicitly, and what we can infer from the scenes we actually witness. Further more, it needs to be pointed out that due to the tight, first person focus, our perception of this world is filtered through the perceptions and knowledge of a 15/16 year old person, who freely acknowledges at the beginning of the story that he (at least at that point in the story) was only granted a limited amount of knowledge about that world due to his "dud" status.

You seem to have some distinct perceptions of the Coven as a group that has limited power and survives on reputation and fear. I agree that they use deception, reputation and fear as tools in maintaining control, but I disagree that their power is as limited as you seem to believe.

The Coven discusses "Binding". You've decided you know what that means. It's entirely possible that you may not be correct. Speaking purely for the purposes of example, "Binding" might well be an operation similar to a lobotomy, permanently excising elements of the body used to control and channel magic. What we do know from the story is that both Agatha and Kelly think of "Binding" as a permanent thing.

You're also making a lot of assumptions about the Necklace they gave Kelly. All we actually know is that they treated it as a trivial thing and that it was fairly powerful magical item (regardless of the fact that whatever is happening to Kelly overpowered it). We've seen that Kelly is stronger than Mary, and we've seen that Agatha is stronger than Melissa, but we have no idea how Kelly compares to Agatha, or to any other member of the Coven, or to the Coven's combined power. We do have EoF's own comment that the Witches are stronger when working together. Additionally, we know that the process of training a Witch is something that takes years, meaning that it's a perfectly reasonable assumption that Witchcraft is just exactly that, a craft, where practice, knowledge and skill play a crucial part in proper execution. Practice, knowledge and skill are things we know that Agatha (and the other members of the Coven) have, and which we also know that Kelly does not. Hence my analogy of the weightlifter and the martial artist.

The Unseen face a lot of risks in this universe, there are a number of groups who dislike them strongly, and are willing to perpetrate violence against members of the Unseen they catch. Violence up to, and including mass murder in the case of the Locke women, as told in For the Fairest. There are plenty of good reasons for the Coven to obscure their actual power levels regardless of whether they're as strong as I believe them to be, or as weak as you seem to believe them to be. Smart people always want their enemies to underestimate them, and the Coven doesn't seem like it's composed of idiots.

Additionally, look at the earlier scene involving Agatha and Mr. McClintock. Mr McC complains, and Agatha counter challenges. Mr McC backs down, not because he's afraid of Agatha's reputation, but because he's afraid of Agatha. No one in Ravencrest wants to piss off the Witches, and many of those people have considerable power of their own and memories that go back hundreds of years in some cases, and they still have a healthy respect for the Witches (at the very least) and outright fear in some cases.