As for a halakhic ruling in the sense of what God would want you to do, it is forbidden to listen to anyone, for the reason that idolatry is forbidden. As it says: “You shall have no other gods before Me.”

Where does a rabbi in his community get formal authority from?

Because they accepted him upon themselves.

Point, it also says, “Do not deviate from their words, neither to the right nor to the left.”

First of all, that refers only to the case of: “If a matter of judgment is hidden from you, between blood and blood, between legal claim and legal claim, and between lesion and lesion, matters of dispute in your gates” — purely judicial matters (or perhaps medical matters as well). And someone who intentionally does not listen to that priest or judge is liable to death. In addition, it is emphasized there that this must be “from that place which the Lord shall choose,” meaning, at a time when the Temple exists.

The central point is that human beings look for someone to lead and guide them, and that is contrary to the spirit of the Torah. According to the Torah, each person is responsible for his actions.

Maybe contrary to the spirit of the Torah (I have no idea where you got that from, but never mind). In your previous comment you claimed that it is forbidden because of “You shall have no other gods before Me.”

I don’t accept the sophistry in your first sentence; “between lesion and lesion” is not a medical matter (since when is the Sanhedrin responsible for healing?) but the laws detailed in the portion of Tazria about lesions. These are entirely matters between man and God — and nevertheless one must follow them. Besides, so what if it only applies when the Temple exists? That proves that it is not idolatry and not contrary to any spirit of the Torah. There have always been ignoramuses and plain people who are not Torah scholars asking their rabbis. “Do not deviate” is only an example of the fact that the Torah gets along just fine with leadership (really the entire Torah is full of this — they asked Moses what to do all the time. And not only in legal matters).

Obviously each person is responsible for his actions. So what? The question is whether it is proper to ask sages.

So let me reveal to you that the spirit of the Torah blows strongly in the direction opposite to idolatry.

The whole matter of the Temple itself, and afterward a king as well — all of that is itself part of that same idolatry that human beings seek. So the Torah and the prophets tried to provide solutions for it, more or less successfully.
As for lesions, that isn’t sophistry. It is the simple understanding that the lesions appearing in the Torah are medical conditions and not magical conditions, as you like to think.
The sophistry is the Torah’s. The Torah forces those who demand sorcerers or kings to go up to the place that the Lord will choose. Let them suffer a little.
Historically this solution did not succeed. So people try other solutions.

Here you are clearly interpreting the reason for the verse. Where do you get all these theories from?
Thank you very much for the psychological analysis (“as you like to think”). I do not think lesions are magic. The fact that they are a medical problem does not mean people turn to the Sanhedrin in medical matters. The only command given in the Torah about lesions is to become impure because of them, leave the camp, etc. A commandment! Not advice for people who like magic. Therefore, someone who goes to the Sanhedrin because of lesions wants to know whether he is impure or not. From here we see that sometimes it is proper and obligatory to turn to the sages.

The rule that if one sage prohibited something, another sage may not permit it was said about a ruling on the object itself, not about rulings in general.

Yishai, I haven’t checked this in detail right now, but it really doesn’t seem so to me.
On the face of it, this may depend on the reasons behind the law: whether it is because of self-imposed prohibition (“shavya anafshei chatikha de-isura”), in which case seemingly it applies to the object, or because of the honor of the first sage, in which case there is no reason to distinguish between object and person. But even according to the first reason, in my humble opinion we find the rule of self-imposed prohibition even in a ruling that is not about the object itself (this too may depend on whether it is based on a vow or on credibility, and it is not conclusive, and this is not the place to elaborate).
And as for what the Rema wrote, that it applies only to that same piece which was prohibited and not to another piece, in my opinion he does not mean specifically the object itself, but rather that this was said only regarding another ruling on the same case about which they ruled, and not a ruling on a different though similar case. In other words, this is a law that deals with halakhic ruling and not with legal instruction (like responsa, which deal with a case, as opposed to the Shulchan Arukh, which deals with general law and does not speak about a specific case).