I recently read in one of the replies that Chabad believe that when Mashiach comes he will replace the Talmud and Torah with Chasidut, is this a common believe among Chabad? Is this the prevalent belief amongst ALL Chasidim? Do the Lubavitchers still believe Schneerson is Messiah and he will somehow come back? And don’t they(Chasidim) deviate from the Torah by the belief that their Tzaddik acts as an intermediary between them and HASHEM and that they do not emphasize the strict observance of Torah or something like that? In short, is the Chasidut a "kosher" movement, as great rabbis like the Gaon of Vilna vehemently opposed it?

Any question can be asked, that is what we are here for; but it must be in a proper way. The Lubavitcher Rebbi is not your school mate and should be addressed as Rabbi Schneurson. To your question, I'm not a Lubavitch Chassid and no authority to answer on their behalf, even so I'm sure there's a mistake somewhere since I can't believe that's what they said or meant. More so the Rambam writes (Melachim 12) that in times of Mashiach the world will be the same world apart from the fact we will have sovereignty over ourselves and we don't know how things will be till they will be. Torah has more than one way of manifesting itself and Chassidut is one of those valid ways. Chabad in particular have many merits in their work to spread Judaism all over the world among many more qualities.