Based on what I've learned in this thread, there is another issue: I think it is very likely that whydunnit and howdunit can be classified as technical 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, hirsutene 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 me, 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 whodunit." An alert listener might say "Do you mean, like a whodunit but with why?" 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 Nineteen Eighty-Four 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 think we can say that "btzptpqvvvvkxz" is not a word yet, but even there I feel I need to add the word yet to be safe. Runcible was not a word until Edward Lear used it in a poem in 1871. Now it is.
