Halacha and legend

We have accepted from the trilogy and the website that the rabbi’s opinion is that it is impossible to truly “study” a legend or the Bible in the sense of receiving instructions or ideas from them that we did not have beforehand.
This is in contrast to Halacha, which has an objective content structure, and the texts within it do indeed contain normative claims that can change things for us (meaning that we must accept as “what the text says” even if it doesn’t fit with the rest).
I give an example from Tractate Nida, which contains a kind of “mixture of authorities”, from Tractate Nida, page 1:
The Mishnah states that a girl’s vows are examined from the age of 11, and a boy from the age of 12.
And in the Gemara, which is the opinion of a rabbi, but Rabbi Ben Elazar disagrees with him and believes that it is the opposite – a boy from the age of 11 and a girl from the age of 12.
The Gemara asks, “49 Darbi, that it is written, “And the Lord God built the rib” – teaches that the woman was given greater understanding than the man. And Idach brought her to the Resh of Lakish, saying, “And the Lord God built the rib”… teaches that the Blessed One, blessed be He, took her from the Lord and brought her to the first Adam, as in the seas of the sea, the rib is called “built “.
We see here that the “accountability” that each side is required to give in a halakhic discussion (which, if I understood you correctly, is one of the pillars that makes the halakhic discussion more “real” than a discussion of the Bible or a legend), can also be given in a completely legendary form, and the verse can be interpreted as making a factual claim rather than a normative claim.
There are other cases like this one (in which, in the ping-pong of “How Everyone Demands,” someone moves on to a legendary sermon), and this seemingly shows that the Sages also perceived the legend as a serious teaching.
I would add that there are often quasi-halakhic discussions on mythological subjects as well (for example, the long discussion in the Sanhedrin about whether redemption depends on repentance, or the discussion on Rosh Hashanah about when the world was created).
Our rabbis have clarified the issue – is there really such a clear boundary line between the fields, and perhaps the reason why agga is so elusive is simply conceptual ambiguity (it is less clear what is being talked about than in “Shor”, “Betulin” or “Einunita Dvarda”), but with the proper depth (or at least if we had all the tools of Chazal’s sermon) can we learn content from it?