Hey, I read the debate and I'll just jump into the conversation if that's alright.
First of all about the huge horned skull, since that's what the topic is about. I see how you debated about it being reminiscent of a Dragon or a Giant, (even though you leaned towards it being the skull of a Chaos Demon towards the end) what if it is a little bit of both? We know that the Stone Dragon, who is a descendant of the Everlasting Dragons has the power to bestow dragon-like qualities to Humans. Which means it's ancestors probably had similar powers, and even stronger ones at that. What if they bestowed dragonoid qualities to a Giant, effectively making him look like that?
Second, I read your theory about Artorias being the Four King, Acidic. And by the Sun, that might be the best theory I have ever read. For that theory to be true though, Ornstein we fight in Anor Londo doesn't necessarily have to be an illusion. We know New Londo was flooded to contain the Darkwraiths. If Artorias was one of these servants of the Abyss, he probably would not flood his own forces.
What if it was Ornstein who made the decision to flood New Londo? He might be real when we fight him in Anor Londo. The captain of the four knights, clinging to an illusion of a Goddess, doing whatever he can to make sure the Age of Fire continues. Just like his God did.
Last of all, if I might add a little speculation myself on top of your theory?
The Chosen Undead kills Artorias's 'flesh' in Oolacile. Then the Chosen Undead proceeds to give his soul to Ciaran, as an act of honor to the fallen hero of whom he also creates a false legend, so Artorias is remembered as a hero. Then with the soul of Artorias, Ciaran proceeds to somehow revive her beloved. Artorias, who was corrupted by the Abyss even before his death while trying to save Sif, is now in a covenant with the creatures of Abyss, to come back to life. These 'creatures of the Abyss' is probably the Primordial Serpents, namely Kaathe, as you suggested.
Realising what she has turned her beloved into, Ciaran mourns until she dies by the false grave. This theory also explains why Artorias's grave in our current time is not at the spot where he died, and explains why Ciaran is near that false grave even though she knew exactly where Artorias died.