Mistake regarding a non-Jew is forbidden because the gentiles are decent now and the reason for the law no longer applies.
So why, with judging, shouldn’t the law also lapse because the reason has lapsed? (The question is about a non-Jew who is a Torah scholar and upright.)

I assume this excitement is out of place. You probably meant to make this comment in the thread about mistake regarding a non-Jew. I’m moving it there, and I’ll answer there. Maybe on the way you can cool down a bit. 🙂

https://mikyab.net/%D7%A9%D7%95%D7%AA/%d7%a7%d7%91%d7%9c%d7%aa-%d7%9b%d7%9e%d7%95%d7%AA-%d7%92%d7%93%d7%95%d7%9c%d7%94-%d7%99%D7%95%D7%AA%D7%A8-%D7%9E%D7%A1%D7%9A-%D7%94%D7%94%D7%96%D7%9E%D7%A0%D7%94-%D7%A9%D7%91%D7%95%D7%A6%D7%A2%D7%94#comment-56428

A clarification has now been posted there, and it became clear to me that the question actually was directed here. Here is the clarification:

What Contradictioooon meant was different, and he commented in the right place. There the Rabbi said that a non-Jew is disqualified from judging, but he did not say that this was only in earlier times, when gentiles were not bound by the norms of civilized nations, whereas today a non-Jewish Torah scholar can judge like a Jewish Torah scholar. In this, Contradictioooon found a contradiction in the Rabbi’s words, because regarding mistake concerning a non-Jew he concluded that it applied only in earlier times, when the gentiles were not bound by such norms. The question is how one decides which law regarding a non-Jew still applies nowadays, and which law depends on his conduct and therefore no longer applies nowadays.

My response:
A strange question (which is why I didn’t believe it belonged here). The disqualification of a non-Jew is not because of his character traits or conduct. He is disqualified by personal status from judging and from testifying. Therefore Meiri’s reasoning is irrelevant to fitness to judge. Similarly, disqualifications regarding marriage have not changed because of changes in gentile conduct (there is no permission to marry a non-Jewish man or woman), and likewise regarding the prohibition of deriving benefit from their objects of idol worship.
Indeed, regarding gentile courts we find in the passage in tractate Gittin that they are accepted because they would not undermine their own credibility, and the medieval authorities (Rishonim) and later authorities (Acharonim) discuss this law at length there, but this is not the place. In any case, it is clear that this does not help in a case where a religious court is formally required (such as conversion and the like). And we explicitly find in Gittin 88 that even when their laws are like ours, the rule still applies: “before them, and not before gentiles.”

Maybe if their laws are like ours it is forbidden to go to them, but if they judge like us, meaning they judge according to the Torah, and all the more so if it is in a Jewish religious court, in the Sanhedrin, then perhaps it is permitted for a non-Jew to judge and he can count toward the quorum of the religious court.

Seemingly, from Rashi on Sanhedrin according to his teacher’s version in the Sifrei on Beha'alotekha, a non-Jew can be a judge on the Sanhedrin, since this refers to the descendants of Jethro through the Kenites and the Rechabites (also timely, since next week’s Torah portion is Yitro):
Rashi on Sanhedrin 104a:
“These are the Kenites who came from Hammath, the father of the house of Rechab” — these Kenites are the descendants of Jethro, as it is written: “And the descendants of the Kenite, Moses’ father-in-law”; and the descendants of Jethro are the descendants of Rechab, as it is written: “the Kenites who came from Hammath, the father of the house of Rechab,” and it is written (Jeremiah 35): “And to the house of the Rechabites Jeremiah said, Thus said the Lord… because you have obeyed the command of Jehonadab your father… there shall not be cut off from Jehonadab son of Rechab a man standing before Me for all days.” And we say in the Sifrei: Is it possible that they were gentiles and entered the Sanctuary, when all Israel did not enter the Sanctuary? Rather, they sat in the Sanhedrin and taught words of Torah, in the name of Rabbi.”