I am also extremely curious about Lovecraft's handling of the Cthulhu Mythos in such tales as "The Horror in the Museum," "The Electric Executioner" and "The Diary of Alonzo Typer," among others.
There was an issue of
Crypt of Cthulhu, titled
Ashes and Others, which brought together the two de Castro originals, as well as Lumley's original for "The Diary of Alonzo Typer" and Hoffmann's "The Lord of Illusion". However, if these are to be included in this proposed volume, then you may want to wait until these are released, rather than tracking down and paying for that issue... which is going, these days, for a hefty price, I believe.
S. T.'s usual comment regarding "The Horror in the Museum" has been, "I fervently hope that 'The Horror in the Museum' is a conscious parody -- in this case a parody of Lovecraft's own myth-cycle." (H. P. Lovecraft: A Life, page 524) By the time S. T. wrote The Rise and Fall of the Cthulhu Mythos, there is no doubt whatsoever in his mind that the tale is a silly parody that Lovecraft infused with ridiculous Mythos tripe out of boredom. I am simply not convinced. Why would an artist who is always trying to do his very best and write serious fiction want to write parodies of that fiction? And yet one must admit that "The Horror in the Museum" is one of the silliest, most ridiculous stories in the Cthulhu Mythos, and certainly unlike Lovecraft's somber and serious fiction under his own byline.
I think that last is the difference, Wilum: This was not under his own byline, and as the other revisions (and some of his letters) would suggest, he felt more leeway in letting his sense of play enter into these productions. He knew that his clients would not object (they might even enjoy it), and the readers of
Weird Tales would be almost certain to miss the farcical elements, given the sorts of stories which garnered so much praise in The Eyrie (the letters column of the magazine). So it was often a way of either injecting a little enjoyment and humor into a task which was frequently uncongenial (at best) and absolutely mind-numbing (at worst... as in "The Last Test"). And, by the time he was writing "The Horror in the Museum", if I recall correctly, he had already encountered William Lumley, who firmly believed in the reality of these various "deities", and of HPL, REH, CAS, and co., as their (unwitting) prophets... which may well be a point he was satirizing in the tale itself.
I am also hoping that, as appendix, the two-volume set will include such things as Zealia Bishop's 1953 memoir, "H. P. Lovecraft: A Pupil's View."
It would be nice to see this piece once again easily accessible... as well as any other such items. But, as I am sure you are aware, one must take anything Ms. Bishop says with a rather large amount of salt... about the same as is the case with, say, Muriel Eddy....
However, I am delighted that such a set is "on the cards", as (despite the serious flaws in many of these tales, even occasionally in the best of them, such as "The Mound") there is enormous room for commentary, criticism, and rich scholarship on this body of Lovecraft's work....