To Jewish believers: Why is it OK for Jews to think Scherson was the messiah or Rabbi Nachmann but if anyone believes Yeshua was...?

Your question is flawed since Jews do not consider it ok to believe that any dead person is the messiah. No one believes that Rabbi Nachman is the messiah and the tiny sect that believes Rabbii Schneerson is the messiah has been kicked out of the community- there Rabbis are not expected, we do not accept there kashrut certifications, they cannot participate in communities and are on the verge of being kicked completely.

As for believing in Jesus, it really doesn't matter what name they use, he has nothing to do with Judaism, has never been part of Judaism, and believing in Christian theology completely contradicts Jewish theology and the Torah. Believing in the beliefs of another religion means that the person is no longer part of the Jewish community and their soul is punished with Kares and permanently cut off from G-d and the Jewish community.

edit: IN response to your updates:

We are saying that the mashiach is meant to achieve everything the mashiach should do while ALIVE. There is no being dead and coming back for a second try. If the mashiach fails to do what he is meant to do while alive, then they are obviously not the mashiach!

Believing in ANY man, god or concept other than G-d is forbidden in Judaism! The mashiach is NOT worshipped! Even when the real mashiahc arrives, he will NOT be worshipped- he is only a man, NOT G-d, NOT divine. Any person who attempts to change even a single letter of the Torah, calls for any law of the Torah to not be obeyed, is NOT the mashiach! And anyone that errs and believes in such a person, believes in or worships any deity, person or concept oither than G-d is denying Judaism, turning away from it and separating themselves from G-d and the Jewish people.

All people are "entitled" to an opinion. But to paraphrase an American politician, they are not entitled to their own facts.

The Lubavitcher Rebbe is not the Messiah simply by virtue of him being dead. (There are other reasons, but I will not get into it now.)

Those who think otherwise, are simply wrong.

A more basic question then is: Is a person who believes the Messiah is Jesus considered an apostate or not?

The answer is "YES".

because there are no rules to making sh!t up and pretending to believe it.....