One thing about the issue of apikursis. We have discussed previoulsy on other threads (where the topic may have been Rav Kook) that it is quite possible that one cannot be an apikores based on his beliefs. What may be required is a life of hefkerus, i.e., throwing away Torah and Mitzvos. The belief is only an excuse to leave the Torah and be porek ol. However, one who fully keeps the mitzvos may not be an apikorus. The Chazon Ish discussed this in a few places, and it is not at all a simple matter.
Furthermore, as others noted, the gemara itself mentions the possibility that a deceased individual may be moshiach in Perek Chelek. So that belief may not be apikorsus in the first place.
Finally, it is highly doubtful that any Chabadnik believes the Rebbe is the Ribbono Shel Olam. They are simply behaving in accordance with the mishna in pirkei avos tzaddik gozer vHKBH mekayem, HKBH gozer, vtzaddik mevatel. Let us not exagerate the extent of what goes on.