In my experience with Chabad (and I have quite a bit), the Messianist movement is overblown by a lot of outside people. It could be that in certain communities there are high numbers of people with problematic beliefs, but I think the average Chabadnik does not believe that the Rebbe is still alive or necessarily is still Moshiach (they almost all think that he could be though).
Also, it’s a little hard to call believing that a dead man can be Moshiach Apikorsus. We might not hold that it’s true, but it’s hard to call it Kefira just because a dead man cannot do everything the Rambam describes in the Sefer Hamitzvos. It is not a belief that we want to let permeate into basic Judaism, but I don’t think we can call it Apikorsus.