I have, sadly, thought about this sort of thing way too much and I’ve come to the conclusion that Dr. Manhattan is rivaled, in all of comicdom, only by the Phoenix.
But, thankfully, Moore realized the same thing Byrne did: having a truly omnipotent character destroys all attempt at story. They took different approaches. Byrne had Jean Grey commit suicide to deal with the power…
Moore had Osterman slowly give away his humanity. But not because he’s omnipotent…because he’s bored. Remember, anything that will every happen to him, even his reactions, are known by him already. Nothing will ever surprise him and catch his true interest outside of his own thoughts and creativity. He no longer cares about defending the United States and humanity and such because they hold no surprises for him. It’s possible, using quantum theory, that not only does he know everything that he’ll ever do but everything that will ever occur anywhere and anytime.
Yes, the mystics might challenge him. But only if he allow(s/ed) it to occur. This is a being who, like the Phoenix, could simply decide that not only do you not exist but that you NEVER existed. He can also take the limitation on that power, having a single point of view, and eliminate it by becoming as many omnipotent beings as he feels is necessary.
In short, The Justice League, all of them, are gone in the first blink of the battle. In combat, which we never see Manhattan in, his powers are such that he can engage the enemy prior to the battle even being considered. He’s like The Midnighter but with the ability to end it literally before it begins.
All the talk of him being able to ‘disintegrate’ his opponents really only displays his lack of thought to the project. And with that limitation on his thinking (he lacks imagination or interest) he still finished Vietnam in a week.
Moore set out to build the ultimate superpowered being and he did so. Then he had to get him offstage for the post-Watchmen era.
Hell, how does one even defeat him? He was reduced to constituent subatomic particles and came back in seconds.
Even if Fate sent him to heaven what would happen? Remember that quote “God exists…and he’s American.” That’s the level of power we’re seeing there.
Again, I think about this way too much.