I don't think they really count as a shared universe, my memory of Lovecraft's correspondence (and, granted, this is from reading a collection of letters back in the mid 1980's, so it could just be failing memory) and an audio interview with Frank Belknap Long is that his younger writer friends would write letters and ask if they could borrow and reuse some of his strange names. Derleth later attempted to knit these pieces together into something larger in his own stories but, I think, this was after Lovecraft's death (again, might be wrong about that).
Derleth came up with occultic correlations like corresponding the Old Ones to the classical elements (Cthulhu-Water, Hastur-Air, Shub-Niggurath-Earth and I think Derleth invented Cthuga to take the empty Fire slot.), etc. But none of this was any kind of "universe" that Lovecraft planned or maintained (or, at least, therein lies the controversy), at best it was just a skeleton of associations/concepts and names he liked.
So, as a hodgepodge or patchwork, "pastiche" doesn't seem too strong to apply to Derleth works like MASK OF CTHULHU.
"Religion is still useful among the herd - that it helps their orderly conduct as nothing else could. The crude human animal is in-eradicably superstitious, and there is every biological reason why they should be.
Take away his Christian god and saints, and he will worship something else..."
H.P. Lovecraft, 1929