Not surprisingly, there was a
thread that dealt with this subject, which revolved around two issues: the ‘loophole’ in the prophesy, and determining which was the fatal blow. Ignoring the question of the prophesy, if you consider that Merry’s stroke to unbind the Witch King’s flesh to his will to be the fatal one, then the Witch King will subsequently die without any further intervention, and all will be well for Merry. If, however, you consider that Merry’s blow only left the Witch King vulnerable to be slain by another, then Merry deals his stoke, the Witch King is unbound, but unfortunately, unkilled, and poor Merry is left with no sword and a bad arm to try and finish him off. As you say, possible, but not likely.
An opinion raised by a number of people in the thread, and to which I agree, is that Eowyn’s blow was the fatal one, for if it was without merit, then Tokein would have explicitly said so. If you accept this, then the latter situation is the case. A subjective assessment, I admit, but one that concords with my impression of the whole scene, in that Eowyn was a principal, and not just an incidental.