Because, you see, it is generally accepted that you can change things a bit if you lampshade or highlight in advance. And what is the central characterisation of MitD? His desire to fit in. To have "friends", he'll accept every humiliation and insult they throw his way. He'll stay in shadows he hates. He won't even leave a place he does't like, despite being perfectly capable of doing so.
Now, into this mix throw in the assumption that he is a protean. A lonely protean, desperate for friends. And everyone he meets has a face with two eyes, and that face is used for communication and recognition and friendship. Then, his constant resistance of his nature to retain a face becomes the central pathos of his existence. This argument, above all others, is what convinced me that the protean has a much higher chance of being the thing Rich picked, because I can see its storytelling potential: an ever-changing creature, struggling to resist change so he can have friendship. And thus making his reveal satisfactory, as Rich expects it to.
Which is not to say I think it has to be the solution. But that if it was, it would most definitely NOT be a cheat, but storytelling 101.