Nope. No im not. You are assuming a lot. Madness is not a new idea. Its a pretty common misconception that psychology is a new thing. It really isnt. It didnt start with mesmer and his theory of animal magnetism. People talk of madness in a context not bringing gods into play just as far back as there are tales of it involving gods. But also, lets say you were right (you arent). If you were right, you would still be wrong. Because what do we call a god plagueing your mind with unhelpful delusions and halucinations? Horrific perhaps? Cosmically so? Yes. A curse laid upon you by an inhuman possibly not fully comprehensible being? Starting to sound like lovecraft and poe yet? Or do hp and poe sound like them? Huh...I disagree. You're extrapolating modern views backwards.
For example, "madness" and "hallucinations" were at various times considered gifts of the gods.
Moreover, your statement, 'I think cosmic horror's actual nethermost core is fear not of the unknown but fear of the recognized fact that something, even if percieved, can still be truly unknowable" happens to be what I wrote.
You know- "ineffable." It's not that it's unknown; it the concept (post-enlightenment) that it CANNOT be known, it cannot be grasped, and it will forever be beyond our ken.
But hey, opinions are just that! Keep on truckin'!