[The Squatter of Amon Rûdh]

I know that mainly the sword was the one that broke the spell that tied the Witch King's self together, but why was it broken when it was only a blade of Westernese?

I hardly think that we can dismiss Merry's sword as 'only' a blade of Westernesse. Clearly there was significant power in this weapon, more so than in most others present on the field. Indeed, we can probably assume that weapons made by the Men of Westernesse embodied knowledge and skills that had been learned from the Elves, and were probably less powerful versions of those used by the Noldor. I do not find it at all difficult to imagine that such a weapon would defeat the Witch King, especially since Tolkien says of the Nazgûl: "Their peril is almost entirely due to the unreasoning fear which they inspire (like ghosts). They have no great physical power against the fearless" (Letter #210, June 1958). Also it is clear that Merry's sword must have been able to break the spell that bound the Witch-King's sinews together, since this actually happens during their encounter on the Pelennor Fields.

Gandalf was indeed present in Minas Tirith during the battle, but I fail to see the significance of his ability or otherwise to defeat the Lord of the Nazgûl. The simple fact of the matter is that he does not do so: Merry and Éowyn share that achievement. It is also entirely possible that the Witch-King is unaware of Gandalf's true race, believing him to be merely a Man. Certainly this would explain the contemptuous tone of his words to Gandalf at the end of The Siege of Gondor.

As with Gandalf, whether or not Galadriel could defeat the Lord of the Nazgûl is irrelevant, since she does not in fact do so. He may well have been concerned about the power of Lothlórien, but I doubt that it is simple fear of Galadriel that prevents his passing near to that place. There are many Elves in the Golden Wood, and to become caught up in a battle with any of them would at the very least have presented a needless delay in his mission. It is also quite possible that he feared the Elves, particularly en masse, but what relevance this has to his death is again unclear to me.