[Inziladun]

Well met, Dakêsîntrah, and interesting thoughts.
Just a few quick thoughts of my own.

Originally Posted by Dakêsîntrah

I do believe it makes logical sense to simply conclude that if the Witch-king and Gandalf ever did engage in a duel, it would be a stalemate.

Originally Posted by Dakêsîntrah

Conversely, I think that in terms of sorcery, Gandalf was stronger; but I doubt Gandalf as a Maia had any chance to really dispose of any Undead Nazgûl.

The Witch-king's vulnerablity to destruction is clear: he was rendered impotent by two mortals. Merry had his Barrow-blade, which had Númenórean spells upon it that apparently enhanced the damage done by it, but Éowyn had nothing but an ordinary sword.
The way I see it, Gandalf had ample power to do away with the WK, or any of the Nazgûl. Think of the manner in which he rescued Faramir from them:

But now the dark swooping shadows were aware of [Gandalf]. One wheeled towards him; but it seemed to Pippin that he raised his hand, and from it a shaft of white light stabbed upwards. The Nazgûl gave a long wailing cry and swerved away..

ROTK The Siege of Gondor

Gandalf was the self-described 'servant of the Secret Fire', and 'wielder of the flame of Anor'. The Nazgûl were said to specially fear fire, and Gandalf's fire would seem to be the worst they could imagine- divine fire from the West.

If mortals could bring about the WK's vanquishment, and Gandalf obviously possessed a power feared and avoided by the Nazgûl, it doesn't seem a question of could Gandalf have destroyed him, but why he failed to do so.

Throughout the books, Gandalf is bound to primarily using persuasion and his own intelligence to accomplish his goals, resorting to 'magic' only in direst need. Even in emergencies, he unleashes only the bare minimum of power necessary to get the job done. Gandalf, and the other Istari, were in Middle-earth not to fight Sauron and his forces militarily, but to direct resistance against him, military and otherwise. They were not allowed cavalier displays of raw power for no reason: that would have lead to their being followed out of fear, not wisdom; and that was the path of a Sauron. Gandalf knew the Men of Gondor and Rohan were still capable of fighting the immediate threat, and kept his power mostly veiled. We see brief flashes of it when he saves Faramir (twice), and earlier when he overthrows Saruman at Orthanc. When the Witch-king threatened to enter Minas Tirith itself, Gandalf blocked the gate, but no more.
What he would have done, had the Witch-king tried to press into the City, could still be debated, I suppose. But my feeling is that under no circumstances would he have allowed the Witch-king to enter, and we would have seen another flash of the White.