I won't be able to answer you in full until sometime tomorrow (I hope I can then, at any rate), but a few thoughts I'd like to throw out there. I don't want to keep on with the argument on evolution too much (even though I think it's very germane to the discussion) but, from my understanding, modern evolutionary theory does take into account such random events -- albeit it cannot, of course, predict future events of such a truly random (from our limited perspective) nature. It also includes a combination of the slow, gradual model with that of periodicity -- evolutionary leaps caused by varying factors. As for the dinosaurs dying out... they didn't; they, too, adapted and evolved, but they ceased to be the dominant species (or genera) of the planet. We see their descendants around us everywhere today, such as birds....
On the subject of Nicholas Roerich and his paintings -- while Lovecraft was tremendously stimulated by them imagistically and imaginatively, he put no stock in Roerich's beliefs, any more than he did those of John Martin, who also impressed him in a similar way. Lovecraft made a very strong distinction between that which was imaginatively stimulating and evocative, and that which was factual or true. This was also the case with hereditary memory, a trope he uses frequently in his fiction, but which he made quite clear he had absolutely no belief in -- found it quite preposterous, in fact, but very stirring to the imagination.
(As for the impressiveness of Roerich's work itself... technically, I feel it is flawed, but I nonetheless find it immensely powerful; much as I do the work of William Blake.)
As for the relationship of Lovecraft's Mythos to the Biblical story of the fall... that began with a letter from Harold S. Farnese, which Derleth picked up on later. It certainly had nothing to do with HPL himself, but chimed in well with Derleth's Catholic upbringing. Even so early as 1937, Clark Ashton Smith was seriously questioning the relationship of the two -- something Derleth had already begun to promote. Fritz Leiber also found this one untenable, as did nearly all those who knew Lovecraft himself. But, as Derleth was long considered (and, to a degree, rightly so) the authority on Lovecraft, and Derleth said it was so, and said it was based on a quote from a letter from Lovecraft (which was some time ago proven to be the Farnese letter mentioned above)... most critics did not question this interpretation. Certainly I didn't, until I had the contradictions drawn to my attention, and then began to read Lovecraft more closely.
You may find the following to be of some interest:
HPLA - H.P. Lovecraft Misconceptions NONFICTION BY DAVID E. SCHULTZ Tolkien, Lovecraft, Derleth - alt.horror.cthulhu | Google Groups Letter to August Derleth From Clark Ashton Smith on 21 April 1937)
At any rate, I'd like to hear more about your conception here; whether or not I agree with it, it's an interesting take on things, and further discussion may help to clarify many points left obscure so far....