This thread was very interesting to read. Of course I could not read all of it. But it was very entertaining, so as to keep me up all night.
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.
One thing that Tolkien replicates is Biblical pneumatology or more closely the nature of angelology.
In Gandalf he seems to replicate to a degree the nature of Jesus Christ in the flesh, or even the angels that intervened in the flesh.
If you agree with my theology, and perhaps even Tolkien's, then we may throw a contrast of sorts into the mix between the natures of Gandalf and the WK, and between the natures of Jesus and Satan.
It seems Gandalf was incarnated in the flesh as Gandalf the Grey, and then *reincarnated later as the White. Much like Christ, in that I see Gandalf the Grey much like Jesus before his death. I see the clothing in white as symbolic of resurrection.
Tolkien would probably agree with me as to the nature of the Deity of Christ, in that Jesus was 100% divine yet 100% in the flesh of men. Same as Gandalf. Furthermore, it is true Gandalf always had the potential to use fully his divine powers. It is not that he was limited or restricted to the use of them while in bodily form. I believe the Valar did not really restrict him to the use of all things divine, just as God had not restricted Jesus during his lifetime. But in terms of both of their missions, their purpose were altogether different than showing off their divinity. If they were to suffer bodily on earth, then it would follow that if they were allowed to transcend the limitations of said physical body, it would have been violated and breached according to the natural laws of the said universe. They were not limited to no divine miracles, they certainly both did many, but they were all in the guise, under the manifestation of what the natural universe allowed.
Now we have the Witch-king, who Tolkien alludes had been demonically enhanced by Sauron. The Witch-king indeed was the Christian equivalent of the Anti-Christ, whom Satan also enhances, rather possesses (depends on how you view theology).
it is certainly correct to portray Gandalf as perhaps temporarily fearful of the Witch-king's presence. I would think Gandalf was more fearful of losing his body in a time where there was a dire need for his bodily presence during the siege of Minas Tirith. I think the WK certainly could have slain Gandalf's bodily form, but there is certainly no way the WK could have defeated Olorin, the Maia Spirit. If Jesus could be tempted, Gandalf could be fearful.
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 Nazgūl. They are neither living nor dead. So who can judge the living spirits, or for the Nazgūl specifically, neither living nor dead? Is it for the Valar to decide, or must that fall on Eru, Mandos? Are they stuck in some kind of limbo, some Abyss?
Someone pointed out why the WK would pull out a flaming sword against Gandalf. I think the WK stood no chance against Gandalf in terms of sorcery, but in terms of melee, the WK would actually stand a chance. But we can all imagine how this magically enhanced sword of the WK might be immune to such deflection or paralyzing spells of Gandalf. We know what happened to Aragorn's sword in Fangorn Forest. I think via melee, WK was confident he could overpower Gandalf in that surprise encounter.
I recall how the WK said he would 'break' Gandalf. I would presume breaking his fleshly body. I think the WK suspected that it was impossible to destroy Gandalf's actual spirit. Surely the WK isn't that stupid to confront Gandalf without the knowledge that he is a Maia. It is Sauron who probably informed the WK of this knowledge, and then possessed it, so it would be emboldened. Without the WK's direct possession via Sauron, you see him flee in the face of Gandalf, Glorfindel, and possibly Aragorn. The WK acting without the direct will of Sauron is just a Man stuck in limbo, whose only real weapon was fear of the unknown, something every man fears in real life, and I am sure was prevalent in Tolkien's own lifetime.
We know that Sauron at the time didn't physically 'possess' the Ring, but he did certainly always possess it spiritually. We say his will was poured into it, but when we speak of will, we speak of Spirit, for they are one and the same. This is Tolkien, who poured his will into the narratives, a will of his that was entrenched in orthodox theology.
To be honest, my Tolkien knowledge pales in comparison to all you folks on here. But I am well versed in Christian theology, and I can already see clearly where Tolkien may be coming from.
Now back to the hypothetical duel between the WK/Sauron and Gandalf: I really do see this as paralleling a hypothetical Jesus versus the Anti-Christ/Satan. You can think back to the temptation of Jesus by Satan. You ask: Why did not Jesus just destroy Satan right there and then while in the body? Well, that was not the Will of God. In like manner, Eru did not will Gandalf to destroy the WK - as I think it is not possible to "kill" a spirit, synonymous to the nature of a Spirit in Biblical Theology. Tolkien does not make mention any possible way for Eru or the Valar or the Maiar to vanquish spirits into nothingness. You ask why God cannot or will not destroy Satan to nothingness, yet his judgment is to be in chains for eternity. You ask why Melkor was not totally vanquished to nothingness, yet he was put in chains into the Void by the Valar. These spirits will always exist, but will be imprisoned in a Realm far outside the Divine or human realms that they cannot interfere.
Now to the breaking of the staff and the prostration of Gandalf in the movie: Yes, it strays from the book, but I see no problem here, considering what I have just said.
I see it quite feasible that the WK should break Gandalf's staff. It's all a symbolic act.
The Staff symbolises Gandalf's stewardship of Middle Earth - not so much a physical conduit to enhance his divine powers. I see the Staff as merely a physical item from that universe that served as a conduit for manifesting divine miracles via the guise of natural law. Much like the cloak of Christ served to act as a physical means whereby one could be healed.
It is a physical manifestation of Gandalf's title, and the WK knows via Sauron that the eradication of anything physically symbolic of his titular power granted while on ME AND Valinor, is worth more than just the physical body. Much like the 'breaking of Jesus' on the cross. A physical 'breaking' that was supposed to show what a failure he is to humanity, and that his mission was also a waste spiritually. And of course, a physically dead Gandalf would be of no immediate use in the siege of Minas Tirith. Hence, I think it is feasible that the WK breaks his staff first, as the body of Gandalf will wait as it is of second importance.
Now, probably Tolkien is turning in his grave at the change submitted by PJ and Co., but Tolkien was not inspired by God to write Middle Earth, so I have no fear to say some changes in the movie are fine, or may be even better.
The Nordic, Finnish or Anglo Saxon et cetera poems are timeless, and many of them like the Kalavala are classic canon material. Yet I find Tolkien replicating them and making changes for his fantasy story. Peter Jackson has his own interpretation, and is in its own right movie canon.
Best Regards