Daasyochid:
If they made a mistake because of some type of conspiracy theory, and there wasn’t a body, etc., you might be right.
However, the reason they think he’s alive is because they think he can’t die, because he’s divine. That’s apikorsus.
That is simply not true. None of them say that. The reason they insist he’s alive is precisely because, like you, they reject the idea that moshiach can be from the deceased, and since they can’t let go of the idea that he will be moshiach they have convinced themselves that the funeral was an illusion, like Moshe Rabbenu’s funeral that the Jews saw at Mt Sinai, and he is really living in 770, hidden from unclean eyes, in keeping with what is found in the Ramban and in the Zohar that Moshiach will go through a period of hiding. That’s crazy, but it’s not apikorsus. They fully agree that the Rebbe is capable of dying, after he has done his job and brought the geulah to physical reality.