I think there is just as much to explore from a hearing person's perspective on deafness as there is to explore the deaf perspective. Why don't hearing people try to write about this perspective? I know of far less books that are written about deaf characters from a hearing POV than I do written from the deaf POV.
In real life, we are always careful to be sensitive of other people's differences and experiences and not make assumptions. Correct? Being a fiction writer doesn't exactly give you the right to disregard this basic rule of tolerance and understanding.
I suppose it does depend on who your audience is and what your intent for writing the story is. If you're writing a thriller, or something of not much substance, then I could care less about using a deaf character the wrong. Such genres rarely depict any experience accurately, so it's not an issue.
I personally deal in writing stories of realism. My genre is meant to be entertaining, but also meant to reveal universal truths about the human experience; to allow us to enter into a world that we wouldn't normally experience. A hearing writer who writes the deaf POV incorrectly not only does a disservice to deaf people, but also does a disservice to humanity and the art form in general.
You're painting with a broad brush here by equating each of your example characters to a deaf character (or even to each other). In the case of Benji, we'll never really know what the experience of being developmentally disabled is like, since the nature of the disability itself most likely precludes the possibility of writing a book about it. Faulkner gave us an amazing and sympathetic hypothesis of the experience of being developmentally disabled. Accurate? We'll never know... One difference between deafness and a developmental disability like Benji's is that there are plenty of deaf people capable of writing about their experience. Why don't we let them handle it?
Using Ethan Fromme and Esther Summerson (I couldn't stand either of those books, BTW. But to each his own) as examples just because they are the opposite sex of their creators is a gross oversimplification of the issue I am trying to bring attention to. C'mon. It is not difficult to enter the mindset of the opposite sex (okay, it is. lol! but it can be done) for purposes writing that character. We interact with members of the opposite sex everyday. It's not a stretch to write such a character.
Yes, I reread the original post, and now I see that as well. Seems it is being written from the hearing person's POV, which I would support, but that still means there are many sensitivities to pay attention to. I'm glad the writer is doing his research.