Ok, so, here's what I have to say.
I agree with downgrading the feat (the ocean is another can of worms), as the clouds created by the OG demon didn't reach Diane's location.
First, the OG Demon appears, casts the spell and causes darkness and clouds to appear. Then we see this (in)famous panel, showing clouds over Diane's location.
But then this other panel comes after.
As we see, there are no clouds anymore after awhile. If those over Diane were actually the OG demon's cloud, why would they go away? Mael's intervention was necessary to dissipate the clouds and the darkness over Camelot, it doesn't make sense that a faraway part of them just went away by itself.
If those were his clouds, the entire sky all the way over Diane's location should have been ALWAYS covered, but it gets cleared on its own, which means those were just normal clouds pushed there by the strong wind, and they went away on their own after awhile. Otherwise, why just a piece of the demon's clouds would separate from the main part and just wander away? It wouldn't make sense.
Also, the clouds over Camelot always come
along with
darkness, while those over Diane were just normal clouds, without any darkness with them (we only see it over the horizon). This further proves that those clouds don't belong to the OG demon.