[Olorin_TLA]

The blades of Arnor found in the Barrow Downs were cetainly enchanted with spells for the downfall of Mordor. Obviously that doesn't eman that one touch and Morodr itself will collapse, but rather that they will be very dangerous to any of Mordor's servants...I believe in RotK it is said that that the forger of Merry's blade would have been proud of its fate and the way it was destroyed; obviously because the WItch King desteoyed the forger's countyr, but also I think because the spells on the blade were made to have an effect on the Nazgul as one of the servants of Mordor.

No man can kill him: did this mean no man in such a position would have done so? I'm not so sure. Perhaps it means more that (I forget who said it) the Elf who said it could forsee that his doom was faroff and not by the hand of a man. Still: it's evident that Merry's strike allowed Éowyn enough time to kill the Witch King, but from reading the texts, it seems to me that Merry's blade MAYBE made the WK's undead, spiritual-almost form more "mortal". Terrible choice of words there, butv there you go. :P (the Witch King doesn't "die", but falls forward in pain: he's still "alive" though) and that allows Éowyn to kill him.
Does her blade wither too?

This may of course not be true, but regardless, I think Éowyn dealt the death blow, but Merry made that possible, at least by saving Éowyn!