Pongo, the implication is obvious that Meriadoc's blade , while not the one to deal the actual death-blow, was absolutely crucial, fashioned as it was by the ancient enemies of Angmar, the Dunedain of the North. Read that line again:
..breaking the spell that knit his unseen sinews to his will.Later on in ROTK, Gandalf says: "All blades perish that pierce that deadly King", the implication being that the WK had been struck by lesser blades previously to little or no effect.
Tolkien makes it clear that this blade was the right blade in the right hands at the right time. Merry breaks the spell, Eowyn deals the final blow. Neither one without the other. No irony intended or required. It is self-evident.
Tolkien creates a specific mythology around the barrow blades; if as you argue, the blades's provenance had so little do do with the final outcome, then why would Tolkien have reason to reference the magical properties of the blade following the WK's defeat? I think the narative intentions are explicit on this count.
I think that the Witch-King wasn't killed by a man because it was his destiny to be killed by Merry and Eowyn. Not because men couldn't physically kill him.
This is absolutely right. The motif of fate runs throughout Tolkien's works. Just as Bilbo was meant to find the ring when he did thus triggering the whole chain of events, the Fellowship were fated to be waylaid on the Barrow Downs and thus find the blades whose use would be crucial later on in the story.