[The Squatter of Amon Rûdh]

[EDIT: cross-posted with Raynor]

...it is quite clear from Tolkien's Letters that he perceived LotR, the book, to be, partly at least, concerned with the "ennoblement of the humble", the weak prevailing over the mighty.

But is that really what Tolkien meant by this? The most obvious parallel to your interpretation appears in letter #131 (Letters p.160):

But as the earliest tales are seen through Elvish eyes, as it were, this last great Tale [i.e. LR], coming down from myth and legend to the earth, is seen mainly through the eyes of Hobbits: it thus becomes in fact anthropocentric. But through Hobbits, not men so called, because the last Tale is to exemplify most clearly a recurrent theme: the place in 'world politics' of the unforseen and unforseeable acts of will, and deeds of virtue of the apparently small, ungreat, forgotten in the places of the Wise and Great (good as well as evil). A moral of the whole... is the obvious one that without the high and noble the simple and vulgar is utterly mean; and without the simple and ordinary the noble and heroic is meaningless.

In his last recorded interview, Tolkien said:

I've always been impressed that we're here surviving because of the indomitable courage of quite small people against impossible odds: jungles, volcanoes, wild beasts... they struggle on, almost blindly in a way.

Another relevant passage appears in Letter #181 (Letters p.237):

[The Tale of Aragorn and Arwen] could not be worked into the main narrative without destroying its structure: which is planned to be 'hobbito-centric', that is, primarily a study of the ennoblement (or sanctification) of the humble.

(emphasis mine)

I note that there is no mention here of victory. Tolkien is pointing out that behind the great events, and often ignored by the more important participants in them, there are hundreds of unregarded individuals who do their bit, and that sometimes a small person in the right place can do more than a more powerful person in the wrong one. That has nothing to do with intrinsic powers, nor with ennoblement. Ennoblement doesn't derive from victory or triumph: it's an internal spiritual process, which can be traced in LR through Frodo's increasing pity for his enemies, or Sam's quest for Elves; even Aragorn's relationship with Arwen. In each case there is some nobler personality or ideal that raises a character up to higher spiritual standing. On this scale, Gandalf, as an emissary of the Valar and the sole Istar to make a decent stab at his mission, is higher than any other character in LR, but it must be remembered that this is not a scale of power, but of sanctity and nobility. Tolkien qualified ennoblement by offering sanctification as an alternative; surely a term that implies a spiritual process, not the defeat of insurmountable odds. Someone may be ennobled in death (as Boromir may be seen to be) and sanctity is unaffected by triumph or disaster. Similarly courage against impossible odds is not the same as overcoming those odds. With the grounding he had in Germanic literature, Tolkien would have appreciated that more than most.

It has been said in the past that Éowyn and Merry's defeat of the Witch-king shows that the weak can defeat the strong, but to me it reinforces Tolkien's statement quoted above. Neither character ought to be where they are at the time they perform the action; they would probably have failed had Merry not in the course of his adventures, come by an extremely powerful weapon, but even so it is only Éowyn's final blow that finishes the job. By chance or providence, two people are in the right place at the right time and with the right weapon to make a difference, but this has nothing to do with their inherent power. It was simply that the application of that power at precisely the time and place they did had a disproportionate effect on larger events. Of course, as obloquy pointed out, neither of them is facing a being of a higher order, however diabolically enhanced he may have been.

As for the Witch-king's ability to defeat Gandalf: whereas Tolkien leaves some uncertainty, it seems unlikely that a Maia could be defeated by a human sorcerer, even one imbued with additional power by another Maiarin spirit. I don't think that the dispersement of power and will required to hold Sauron's armies together allowed him to put enough of his native force into one of his servants as to enable that minion to defeat a fellow Maia. It seems to me that Tolkien realised this, and so chose to have the chief Ringwraith instead confront two weaker characters for greater dramatic tension, rather than simply show him being swatted by Gandalf. The tension arises from the fact that nobody on the battlefield apart from Gandalf can be guaranteed to face the Nazgûl lord successfully.