While there may be other opinions, here is what I have seen/heard from Rabbi Yehuda Herzl Henkin, Rabbi Hershel Welcher, and others:
Category A. Many Lubavitchers believe that perhaps Rabbi Schneurson was a candidate to be the messiah, but for whatever reasons, God chose for it not to work out that way. Such Jews are easily 100% kosher, and in fact that was basically Rabbi Akiva's theology post-Beitar. ("Bar Kochba could have been, but it didn't work out.")
Category B. Those that believe that Rabbi Menachem Mendel Schneurson isn't really dead, or that he gets a second coming to be the messiah, are wrong, and this belief is very dangerous as it can spin into all sorts of other problems. As such, the Rabbinical Council of America does not want members with such a belief. Some would argue that it's best to avoid associating one's self with people (or at least communities or institutions) who believe this. Rabbi Shach was concerned of this danger even while Rabbi Schneurson was alive, and called for people not to associate with such believers. However, such a belief is not so wrong as to void one's halachic status as a witness, or invalidate a Gett, meat, wine, or cheese.
Category C. Unfortunately, there are those who took the next step and blurred the line between Rabbi Schneurson and God. Such a belief (e.g. referring to the rebbe as "above all space and time", or "he controls what happens in this world"; or more simply, "we're bowing to a picture of the rebbe because he totally nullified himself to God") would render one's Gett, conversion, cheese, meat, and wine all not kosher.
So this leaves us with the contemporary questions:
What's the best way to approach Category B Lubavitchers?
How many Lubavitchers are in Category C? And what category should I assume (or fear for) if I have incomplete information?
I believe you're confusing some of the rhetoric between those questions.