“Why is viewing that the rebbe will be Moshiach pasul them from being part of a minyan?”
I believe you misunderstood (assuming you’re referring to what I think you are). That poster was referring to the practice in Chabad shuls to count non-religious people. A person who is publically mechalal Shabbos b’meizid is pasul l’aidus according to seemingly every shittah other than Chabad. This point is brought up far less than all the other for some reason. I suspect due to quasi political correctness reasons.
By the way, about the sukkah, please see recent threads in the CR for thousands of posts debating the topic.