Testing sincerity is not the same as tricking them.
The Rabbis. The Messiah is only a prophet and prophets don't have the capability to contradict the Torah, except on a temporary basis. So generally, if the Messiah had an opinion on the Law, it would only be in his capacity as a Rabbi as well. And the rule is that we follow the majority opinion in Jewish Law.
Also, we don't capitalize pronouns for the Messiah since he is not G-d.
G-d is unchanging and we already have the rules of His conduct.
It depends on the situation. There is no Law to accept the Messiah as being the Messiah. For that, we understand that the Messiah's messiahship will be so obvious having fulfilled all the prophecies, that no one would reject him. To the extent that if it was still possible to reject him, that wold call his messiahship into question. So if the majority of Rabbis don't accept the Messiah, than that means he's not the messiah. If one Rabbi doesn't accept the Messiah and all the others do, than that means the Rabbi might be suffering from mental illness.
If you're talking about with regards to the Law, then again that depends where we're holding. If it's after the Messiah is running the Sanhedrin, then it's forbidden for any Rabbi to go against the Sanhedrin at any time. If it's before then, then there is no requirement for any Rabbi who has been tested as competent in deliberating in Jewish Law to follow the opinion of a contemporary Rabbi.
Of course we believe that G-d tests man too. But it is not through trickery.
Deut. 17:11