I loved your idea of the heart of darkness sounding like Sousa. "What's that noise?" Margaret Dumont asks in Duck Soup when a bit of Sousa comes blaring out of a radio that's not supposed to be playing at all. Groucho cups his ear and says, "Sounds like mice." And of course you're right about the Twilight Zone effect in much of King's work. It's not just that the stuff comes out of the Twilight Zone; it comes out and goes back in and stays there. This is particularly true of Mike Noonan's musings about his telepathic abilities. Not the abilities, which are often creepily rendered, and feel like real hauntings, but his thinking about them, as in "I was in the zone, deeper than I'd ever been, down where the town's unconscious seemed to run like a buried river." This is the right idea, but too clearly spoken as an idea--a good example of what you describe as a failure of vagueness, itself a very interesting critical thought, with all kinds of applications. Calvino says somewhere that vagueness can be a form of precision--if vagueness is precisely what is needed.
But let's talk about genre, which is the trickiest idea in the air here. The remark of mine you quote was just a swipe at all those people who are snobbish about the notion of genre itself, and you are certainly not one of those. But I do think the difference between getting a genre to work for you and just sadly recycling it is often a very close call, and I'm not sure it has to do with conventions being rendered organic or not. That is, it may have more to do with the current life of the conventions than with the artist's sensibility. Maybe he or she just has to know how not to get in the way of live conventions, and how not to choose dead ones. In Bag of Bones, the convention of the house with a life of its own ("I think houses live their own lives along a time-stream that's different from the ones upon which their owners float") is obviously pretty ancient and not all that exciting, but still seems healthy, good for plenty of movies and novels yet. The convention that says this scary story is just like an old scary story ("My mind kept trying to replay an old episode of Alfred Hitchcock Presents") has surely gone squishy underfoot.
Is there a way of talking about King's remarks about fictional murder as pornography without giving the whole plot away? Or is it OK to give the plot away?