Imagine a scenario where a king orders his subjects to sort unlabeled boxes of two types of objects into each of two rooms. The subjects are forbidden to open the boxes but instructed instead to seek the guidance of the king's appointed elders as to which boxes contain which objects.
The elders use various sophisticated, best efforts means to guess at the boxes' contents. Occasionally, the elders may change their minds as to which are the contents of a given box, such as in response to new evidence or just a fresh perspective, and that box will then be reclassified and sent to the other room.
Contrast this scenario with one where the boxes are known to be empty and the king has instructed his servants to fill the boxes with whatever the elders think are in them. Wouldn't the wise of the elders just acknowledge that the boxes are empty and not waste time trying to pretend that the evidence points one way or the other?
I think therefore that it's pretty clear that the latter scenario does not at all represent the halachic process. Rather, the wise King knows that the subjects are human and only requires them to listen to their elders, as is reasonable. Likewise, He knows the elders are fallible humans as well, and only requires a level of effort to guess at the contents. Practical halacha thus follows the ruling of the Sanhedrin, provided one's knowledge is no greater than their own (see e.g. Horayoth 2a-2b where a student who follows the guidance of the Sanhedrin, knowing they are in error, has to bring his own personal offering for his mistake in listening to them, and is not atoned with the communal offering they bring for their error, which does indeed atone for the error of the unwitting masses.
This idea is what is meant in the passage in Bava Metzia:
עמד רבי יהושע על רגליו ואמר (דברים ל, יב) לא בשמים היא מאי לא בשמים היא אמר רבי ירמיה שכבר נתנה תורה מהר סיני אין אנו משגיחין בבת קול שכבר כתבת בהר סיני בתורה (שמות כג, ב) אחרי רבים להטות
Rabbi Yehoshua stood on his feet and said: It is written: “It is not in heaven” (Deuteronomy 30:12). The Gemara asks: What is the relevance of the phrase “It is not in heaven” in this context? Rabbi Yirmeya says: Since the Torah was already given at Mount Sinai, we do not regard a Divine Voice, as You already wrote at Mount Sinai, in the Torah: “After a majority to incline” (Exodus 23:2). Since the majority of Rabbis disagreed with Rabbi Eliezer’s opinion, the halakha is not ruled in accordance with his opinion.
The Torah was given over with rules regarding the determination of practical halacha. These rules include following the majority. They do not include following a bat kol. As per the ruling of the King, even if the bat kol is assumed to describe the actual contents of the box, its testimony is inadmissible in the court of halachic determination. If the sages, however, change their minds, they may then require a par he'elam davar based on their new opinion. (And perhaps at times another one if they change their minds again.)
