We are not allowed to derive Torah Law from the prophecy of anyone other than Moses. A Midrash on Mishlei can never overrule the accepted majority opinion that Gentiles are not allowed to add to their 7 commandments. A Sanhedrin that contradicts the majority opinion is heretical. I gave a specific example of heretical Sanhedrin courts in a previous question.
The terms B'nei Noah, children of Noah, Gentiles and non-Jews are synonyms. You probably rely on Rambam's ruling in Laws of Kings 10:10: "We should not prevent a Noachide who desires to perform one of the Torah's mitzvot in order to receive reward from doing so, provided he performs it as required."
His reward in this context refers to the practical benefit he receives. However, if a Gentile observes any of the Jewish commandments from the Torah as a religious obligation (even if he does so from a desire to receive a spiritual reward), this is forbidden based on the prohibition of adding a commandment, and there is no spiritual reward to be derived from this.
בס''ד
1. There is no "majority opinion" that bnei Noach must forever observe only 7 commandments. There is not even a minority opinion to that effect.
2. Deuteronomy (17:8-13) commands the Jews to appoint a Sanhedrin that will convene at the Holy Temple in Jerusalem. In Deuteronomy, we are commanded to abide by the Torah rulings of the Sanhedrin. The Sanhedrin determines Torah law. From ספר דברים Deuteronomy (17:10-13):
10 And you shall do according to the word they tell you, from the place the Lord will choose, and you shall observe to do according to all they instruct you.
11 According to the law they instruct you and according to the judgment they say to you, you shall do; you shall not divert from the word they tell you, either right or left.
12 And the man who acts intentionally, not obeying the kohen who stands there to serve the Lord, your God, or to the judge that man shall die, and you shall abolish evil from Israel.
13 And all the people shall listen and fear, and they shall no longer act wantonly.
You will notice that anyone who intentionally disobeys the Sanhedrin rulings faces the death penalty.
3. You mention heretical Sanhedrins. The Mashiach (Messiah) will rebuild the Third Temple and the Sanhedrin that will be appointed in the Messianic era and in the world to come will not be heretical.