Based on what I've learned in this thread, there is another issue: I think it is very likely that <em>whydunnit</em> and <em>howdunit</em> can be classified as <em>technical</em> words, that are understood and used for communication between people within some relatively narrow field. In this case I suppose they are used by mystery fiction fans.
Although big dictionaries go a long way to include technical terms that a lay reader might encountered, they all draw a boundary and acknowledge that they omit specialized technical terms.
For example, <em>hirsutene</em> is showing a red line underneath it and isn't in m-w.com, but a Google Books search shows that it occurs, and since the books it occurs in are written in English, I consider it to be an English word. It communicates nothing to <em>me</em>, however.
Miriam, I assure you that if you were to say in conversation "I just read a great whydunit the other day," ordinary English speakers would look puzzled. Then they might brighten up and say, "Oh, you meant to say <em>whodunit." </em>An alert listener might say "Do you mean, like a <em>whodunit</em> but with <em>why?" </em>In writing, most would realize that it was not a mistake and would guess the meaning correctly. You could use it in casual conversation in a way that half-defined it and be easily understood: "I just read a great mystery the other day, more of a whydunit than a whodunit."
I feel sure that every field of genre fiction has a technical terminology. Get a couple of science-fiction fans arguing over whether <em>Nineteen Eighty-Four</em> is science-fiction, or where to draw the line between fantasy and science-fiction, or between just plain science-fiction and "hard" science-fiction.
Is it a word? Is that even a well-defined question? Maybe. I <em>think</em> we <em>can</em> say that "btzptpqvvvvkxz" is not a word <em>yet,</em> but even there I feel I need to add the word <em>yet</em> to be safe. <em>Runcible</em> was not a word until Edward Lear used it in a poem in 1871. Now it is.