“And yes, it is k’fira and apikorsis to believe that a dead man can be Moshiach.”
Nobody believes in a second coming of the Rebbe. Those who still maintain that he is moshiach are those who maintain he did not die, but is rather in a state of “concealment” waiting for the appropriate time to reveal himself. As I and numerous people have already written (as does Rav Feldman in the letter that was linked here) stupidity does not equal apikorsus.