Although I do not believe that PJ and the other screenwriters quite realized that they had made this little mistake, it could be argued that the "release" of the Witch King refers more to his finally revealing all of his power, as opposed to his having ever been to Middle Earth. I'm sure that Sauron had his own plans to "surprise" his enemies by finally alowing the Witch King to reveal himself. Aragorn acted in a much similar way. He did not "introduce" himself (or was not "released") as the true King of Gondor until he looks into the Palantir. But he was still around before then, fighting orcs and other evils, though he had yet to claim himself.
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The world is a great book, of which they who never stir from home read only a page
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