Utterly Mad

The funny thing is, Cthulhu is a fairly minor power in Lovecraft. He showed up directly in only one story. Lovecraft himself titled his work "The Yog-Sothoth Cycle", because Yog Sothoth was a much bigger player than Cthulhu. Also, if you are doing Shoggoths as a black screen portion, why not have the Colours Out Of Space appear as a massive graphical clusterfuck, like trying to render all colors at the same time.
The Poe style protagonist would be a better choice, as a lot of Lovecrafts protagonists were crazy, narrow minded, and horribly Xenophobic to begin with. Just going with a depressed crazy guy might be better. Also, cats have to show up, because Lovecraft loved cats.