What exactly do you mean when you say "speculative fiction"? Are science fiction and fantasy both covered under that umbrella?Yes. Anything that is not realistic, as in, it couldn't happen in this world. It includes science fiction, fantasy, horror, magic, supernatural, super heroes, alternate histories, etc.
Also, if someone wrote an adult speculative fiction book for the general market, would an LDS publisher even consider publishing it? (I'm thinking kind of along the lines of Fablehaven and Leven Thumps, where the books were published by an LDS publisher, but are still known nationally.)Yes. At least, I'd hope they would.