Camoran's speech is rubbish, but it isn't entirely unfounded. He goes on the assumption that everything Padomaic is Daedric; but he forgets that Lorkhan helped create Nirn, so he is actually an Aedra. I suppose the speech sort of justifies itself by saying that Nirn is also Daedric, but it still doesn't quite work.
anyone who would knowingly help a divine being whose sphere is destruction invade his own world can't be sane
Oh, I think Mankar is sane. He didn't just help Dagon - the Mythic Dawn were necessary to open the Oblivion gates. The whole thing could have been his idea, and he definitely had a lot of power. Jearl's orders suggest that he was the commander of the Daedric forces, and that Dagon just gave him what he asked for.
His objectives were not to destroy the world, but to have mer, who ruled Tamriel in the Mythic Era, and the Daedra, like in the Dawn Era, on Nirn. The fact that he constantly made references to Ayleids, and could possibly be Ayleid, should make this pretty obvious, even if the name of the Mythic Dawn doesn't.