The way I've seen the Mythos described comes mostly from Lovecraft scholar S.T. Joshi's "The Evolution of the Weird Tale" and various of his essays. He largely frowns upon the mythos (he can't refer to it without quotation marks) and its scattershot attention to and alignment with its inspiration and source material in Lovecraft's own work. He's not the only one to believe Derleth's many posthumous "collaborations" with Lovecraft were largely self-serving pastiches of questionable quality that took advantage of Lovecraft's name for personal gain.
Joshi makes a point of the difference between encouragement and the level of contribution others such as Derleth might claim or imply to their own works and fictional worlds.
For instance, Lovecraft and his circle traded little things like names back and forth. Lovecraft refers to a cult of "Klarkash-Ton" as a tribute to Clark Ashton, for instance, and introduces a "Robert Blake" into a story as a tribute to Robert Bloch, who himself names a character Luveh-Keraph. But that is fun between friends. Lovecraft would never, for instance, have embraced the turning of his concepts toward an embrace of religion and a human-centered teleology.
Lovecraft is a little like Bruce Lee, in a way. Lee created a new and extraordinary martial art he did not set down systemically, and when he died young it was claimed by everyone to their own purposes. Within a generation it was unrecognizable.
Bloch was 16 years old when he first contacted Lovecraft, and knew him, via correspondence alone, for about four years before he died. We could say he was a contemporary of Lovecraft technically, but really Lovecraft was more an inspiration and mentor.
I've got Daniel Harms' "The Cthulhu Mythos Encyclopedia" on the way (been back ordered for a month or so by now), and will be interested to read his takes on the mythos. I've got plenty of mythos stories in the "to be read" backlog, too. I'll be getting more of S.T. Joshi's critical studies of Lovecraft and weird tales as I get my out-of-control kindle spending more in line with what I can really afford.
