The Shoggoths were not, or at least were not merely, the creations of the Elder Things
Part of Danforth's insane ramblings at the end includes "proto-shoggoths", hinting that their origin may be much stranger than suggested earlier in the story, and directly related to the other horrors that he mentions, like the Colour out of Space and Yog-Sothoth. They may have instead been larvae of the Outer Gods, or similar beings of otherworldly origin that the Elder Things bound in Earthly matter to control them and turn them into slave labour. Later they either forgot or deliberately suppressed the fact, and their mundane science was no longer able to replicate the amazing, physically nigh-impossible versatility that their earliest "creation" demonstrated. Losing the information of their origin, they also forgot the occult secrets needed to truly master them, and the rest is bloody history. What Danforth saw beyond the Mountains of Madness which drove him to insanity...
...Was actually a massive amount of shoggoths assuming the shapes of things they've stored from memory across the time of their existence, contained by the arctic snow and ice, yet very much so alive and slumbering. The fridge horror of what might happen if the snow and ice ever melted and if anyone would wake the mass up is what drove him completely nuts as consequence.
At the end, Danforth saw Adam.
Going mad seems to be a recurring theme there too.
At the end, Danforth saw...
...nothing. But not just nothing; he saw an emptiness, a void, so deep that there was no colour, no space, no light...simply nothingness. And because the human mind can't comprehend such emptiness, the slightest glance drove him mad.
