Re: re
L4P - you're so close, and I assume from your references only being from the movies that you haven't read the books?
Anyway - Tolkien makes this quite clear, in the books, after the Witchking is dead.
Basicaly, it is the sword from Barrowdowns that Merry took with him - Pippin & Sam also had swords from the Barrowdowns - that were made by the men of Westernese during their long battle with the Witchking of Angmar.
These swords had been created specifically to break the Witchking's spell and make him vulnerable to attack.
So basically, when Merry stabbed the ML in the leg with this blade, his spell was broken and became vulnerable to the attack from Eowyn.
Could the KotD have done as Eowyn? I see no reason why not, especially if he had a trusty hobbit (or anyother type of creature) stabbing the ML with such a blade.
Could the ML have killed the KotD? Nope, as the KotD is Dead.
8o