With regard to the description of Shub-Niggurath - first of all, I would submit that S.N. is worshipped as a fertility god (goddess) by said cults - It is the perfect description for a Chaotic Evil Fertility god, if you ask me. I don't see it so much childish as graphic - so to each their own.
Besides, how do we know it doesn't impregnate male cultists, too?
Second, as to the pedigree of the Necronomicon - GEEK ALERT - It was, respectively, Written in original by Abdul al Hazred (Abd al'Azrad - "The servant of the Ravager"), then translated sometime between the 8th and 10th centuries to Greek by Philetas(sp?), then to Latin in the 13th century by Olaus Wormius, then to English Twice by Dr. John Dee (court wizard for Queen Elizabeth I), and another Englishman whose name I can't remember, then expurgated into the Sussex Manuscript in approximately the 17th Century. This acccording to the CoC version 5.5 game from Chaosium.
Finally, as for Lovecraft's vision for his stories - Lovecraft had no "continuity" vision. As was common for authors of "weird tales," he swapped story ideas and elements with friends and other authors regularly.
If Lovecraft had one "vision" it was this: That the universe is a very scarey place, and beneath our facades of comfort and sustenance, there is a harsh, cold, unfathomable place waiting to strip us of all we hold dear. This actually figured in very heavily from Lovecraft's life-lessons - his father left(died? abandoned?) he and his mother at a young age, and he stayed until his early teens at his grandfather's almost Victorian/Palatial style house in Providence, RI. His grandfather made some bad business deals, and he had to move, losing "his" beautiful study and library loaded with old books, and it was his sanctuary where he could lock out the world. He spent the rest of his life being a poor gentleman writer, living from feast to famine (more famine than feast) on his stories. Is it any wonder than unfathomable horrors that considered humanity beneath notice played a huge part in his stories?
He tied this in with the idea that Science was a very cold thing, that unchecked would devour all that was artistic, thoughtful, and beautiful in human beings. He was (I think) a life-long atheist, but his ideas had almost the trappings of humanism. I just can't bring myself, unlike Gary, to call him a humanist, though.
Now, about the price of Tea in China...