Believing that a dead Rebbe is moshiach is idol worship,
No, it is not — but claiming that it is, is itself a symptom of idol worship. The only way you could possibly come by such a view is if you are not a Jew but a Xian, and therefore to you the word “Messiah” means a god, r”l. Xians worship the person they call “Messiah”, so it’s very important to them that he is the Messiah. Jews, actual Jews, believe that the moshiach (lower case M) who will soon come will be an ordinary person, of flesh and blood, and no more an object of worship than any tzadik. He will be a king and a religious leader, not a divine entity.
And while we expect that he will be someone who is currently alive, there’s no actual reason he can’t be someone who is not currently alive but will be then. We already believe that techiyas hameisim will occur in stages, and the greatest tzadikim, who will be needed immediately, will return then. So there’s no reason one of those early risers could not turn out to be the moshiach. I don’t expect that to happen, but I wouldn’t be shocked if it did. Maybe even Dovid himself (though that would contradict a gemoro that says the moshiach will be descended from Shlomo; but maybe the moshiach will not pasken like that gemoro).