Regarding your first answer—this is a novelty, because according to Maimonides, in general, what the Sages derive via the thirteen hermeneutical principles is rabbinic, whereas according to the Raavad, as brought in Rashba and Ritva in Berakhot there, it is indeed Torah-level. But not that it was newly introduced in his generation; rather, he explains that even according to the Sages it is Torah-level and everyone had always said it, only Ben Zoma found a source for it.

That is, nobody can maintain that the Sages can create a new law that did not exist until then and that it would be Torah-level—I’m not talking about “do not deviate.” So note that Maimonides not only did not count the commandment of remembering the Exodus from Egypt at night in his enumeration, he also did not say that it is Torah-level, only that it is a commandment (Laws of Shema 1:3), which means rabbinic according to his aforementioned view (see also the introduction to his Commentary on the Mishnah).

In any case, the Raavad’s answer is untenable, as brought there in the medieval authorities, because there are explicit Talmudic passages showing that there really were people who did not mention the Exodus from Egypt at night at all. So Ben Zoma really did innovate this.
And therefore your second answer also falls away—or at least is not necessary—because we do in fact see that until his time they did not say it, and therefore “I did not merit” is to be taken literally.
In any case, I also didn’t really understand how you arrived at this interpretation, since the wording “I did not merit that it should be said” means literally that it was not said at all, not merely that there was no source.

But let’s go with Maimonides’ approach, that the Sages can derive laws via the thirteen principles. According to Maimonides that makes them rabbinic, and according to your approach in the first answer it makes them Torah-level. What is the meaning of this strange thing? The Holy One, blessed be He, did not give this commandment—any law they derive—to Moses, and they did not practice it until the generation of the Sage who derived it through the thirteen principles.
So what is this strange mechanism? I understand that the Sages have the power to add decrees, ordinances, and safeguards as needed, or to institute rabbinic commandments according to the circumstances.
But to invent some new commandment, not because of some concrete need, but only because he found some hint to it in a verse, a hint that until now no one had found? Not Moses himself, not Joshua, not the Men of the Great Assembly? That is not something reasonable. What was that Sage thinking? And add to that that this sage’s textual discovery is not something brilliant, nor something that cannot be rejected—even fairly easily.

An absolute majority of Torah-level Jewish laws were developed over the generations. See Rabbi Akiva’s exposition of “and she shall be in her menstruation” in tractate Shabbat. Maimonides also writes in his letter to Rabbi Pinchas the judge of Alexandria that most of the derashot we have, apart from perhaps three or four, are creative rather than preservative. And that is also what one sees in the introduction to the Mishnah and in the second principle.
When the Sages say that everything was given by the Holy One, blessed be He, at Sinai, that is a normative statement—meaning that this is how we are to relate to Torah-level laws that were developed later—not a historical statement.
And see Havot Ya'ir, section 192, that even regarding laws given to Moses at Sinai, about which Maimonides wrote that no dispute ever arose, many disputes did in fact arise. And Nachmanides and his students showed that even gezerah shavah, which a person may not derive on his own unless he received it from his teacher, often generated new laws.
And the Netziv, in Kadmat HaEmek, showed that when the Talmud says “it was learned as a tradition,” Rashi explains that this means a law given to Moses at Sinai, whereas Maimonides explains that it means a newly developed law whose source has been hidden from us. By the way, in light of what I wrote, this is not necessarily a dispute.

Maybe you wrote from memory, because in that letter it says the opposite: that all the laws innovated by the Sages are rabbinic except for 3–4. If it were like you say, it would be much harder for me to understand, and again as I wrote in the message above, nobody says something that strange to that extent.
But as stated, even if it is only rabbinic, my question above still remains. What is this strange mechanism, and what was the amora thinking?

By the way, the statement that the whole Torah with all its details is from Sinai—Maimonides already explained in the introduction to his Commentary on the Mishnah that this refers only to what was actually given, such as accepted interpretations transmitted from person to person, or laws given to Moses at Sinai. But truly not everything was given; things derived through the thirteen hermeneutical principles were not given at Sinai.

I most definitely did not make a mistake.
Maimonides says that most derashot are creative rather than preservative, and according to most of the medieval authorities, laws derived from derashot are Torah-level.
There is no dispute that the derashot are not from Sinai. Maimonides’ innovation is only that their status is rabbinic.

But you mixed together two views.
I agree that according to Maimonides, most if not all of the laws are innovations, but they are rabbinic.

And I agree that there are those who hold they are Torah-level—you wrote that this is the majority view; I’m not aware of that, on the contrary, what I know is otherwise, and I’d be happy for a source—but they do not hold that they are innovations, as with the Raavad I cited above.

And again, even if it is only rabbinic—and all the more so if its force is Torah-level—my question is: what is this strange mechanism, and what was that Sage thinking?

I’m done. Everything was explained.

Except for what I asked.
In any case, thanks.