The only thing he needed to be physically present for during the Gathering was releasing Lloyd from his cell, everything else was done through dreams and hallucinations. He isn't near as omniscient as he'd like to believe (since he doesn't even notice the pyro screaming "My life for you" riding into Las Vagas), and the shapeshifting into a corvid is done, with only one exception (where he was panicking) in psychic contact. He only turned into a devil to screw with Nadine and increase his fear factor, not because he actually is a demon. And last but not least, while in the book Flagg is still plotting in the end, but in the miniseries, a mere burst of intense heat and radiation destroys him.** Not really...he turns into a crow and flies off. Mother Abigail's God is probably something close