Avira,
In the last few posts I’m only discussing the Moshiach issue, nothing else. And I can certainly buy that the meshichists are meshugoim, even though that belief doesn’t make them an apikorus.
Now regarding Rav Menashe Klein zt’l, you can read his teshuva that you’re referring to in Mishne Halachos 17. He calls for the Yechi Melech Hamoshiach to stop, and he spends two pages calling on Lubavitchers to appoint a new rebbe. He does attack the idea that the Lubavitcher Rebbe will be Moshiach, but he doesn’t call it apikorsus. What he calls apikorsus is the Bizuy Talmid Chochom that he thinks the radical meshichists cause, when they insist on rubbing their mishugas in everyone’s faces. They turn the Rebbe into a joke, and that is what Rav Klein says is their apikorsus, by being mevaze the Lubavitcher Rebbe.
It is also noteworthy to mention that a few years after the Lubavitcher Rebbe passed away Rav Menashe Klein publicly said that the meshichists are not apikorsim, and that it is not forbidden to have the belief that the Lubavitcher Rebbe is still Moshiach.