By what authority, and roughly when, did Judaism concede that adoption of Noachide laws was an acceptable alternative to conversion?

Question

I. Introduction.

According to:

1. B. Shabbat 145b-146a. "Rabbi Yoḥanan then explained to them: Why are gentiles ethically contaminated? It is because they did not stand on Mount Sinai. As when the snake came upon Eve, i.e., when it seduced her to eat from the Tree of Knowledge, it infected her with moral contamination, and this contamination remained in all human beings. When the Jewish people stood at Mount Sinai, their contamination ceased, whereas gentiles did not stand at Mount Sinai, and their contamination never ceased. Rav Aḥa, the son of Rava, said to Rav Ashi: What about converts? How do you explain the cessation of their moral contamination? Rav Ashi said to him: Even though they themselves were not at Mount Sinai, their guardian angels were present, as it is written: “It is not with you alone that I make this covenant and this oath, but with he that stands here with us today before the Lord our God, and with he that is not here with us today” (Deuteronomy 29:13–14), and this includes converts."
2. B. Yevamot 103b. "The Gemara answers: He implants filth in her and contaminates her, as her body accepts his semen. As Rabbi Yoḥanan also said, based on his understanding that the serpent seduced Eve into having sexual relations with him: When the serpent came upon Eve, he infected her with moral contamination, and this contamination remained in all human beings. When the Jewish people stood at Mount Sinai their contamination ceased, whereas with regard to gentiles, who did not stand at Mount Sinai, their contamination never ceased."
3. B. Avodah Zarah 22b. "...as Rabbi Yoḥanan says: At the time when the snake came upon Eve, at the time of the sin of her eating from the Tree of Knowledge, it infected her with moral contamination, and this contamination lingers in all human beings. The Gemara asks: If that is so, a Jew should also be suspected of engaging in bestiality. The Gemara answers: With regard to the Jewish people, who stood at Mount Sinai and received the Torah, their contamination ended, whereas in the case of gentiles, who did not stand at Mount Sinai and receive the Torah, their contamination has not ended."

II. Questions.

1. By what authority, and roughly when, did Judaism concede that a non-Jew's adoption of Noachide laws was an acceptable alternative to full conversion to Judaism?
2. By what authority, and roughly when, did Judaism decide that the moral contamination inherited by non-Jews was abated by their acceptance of the Noachide laws?
3. By what authority, and roughly when, was it decided that the moral contamination inherited by non-Jews no longer exists at all?

Answer 1

There seem to be a few underlying assumptions which need to be clarified.

All of mankind is obligated in the 7 Noahide laws, as the gemara teaches (Sanhedrin 56a-b.)

At Har Sinai, the Jews accepted a covenant with G-d, whereby Jews committed to keeping G-d's commandments and will, and G-d committed to making the Jews a "priestly "kingdom" and a "sanctified" nation. (Exodus 19:3-8)

We don't find any obligation for the Non-Jews to similarly accept this covenant. (There are sources which indicate that G-d perhaps offered other nations the chance to receive the Torah, but I'm not aware of any source which indicated any sort of obligation for them.)

Thus the first question is slightly misconstrued- there was never a "concession" that accepting the Noahide Laws is acceptable instead of converting; when the Jews underwent the national conversion which was at Har Sinai, there never was an expectation for the other nations to follow suit.

(I'm avoiding the point about pre-judaic religion. There is discussion of how all the nations of the world could have had the same relationship with G-d as the Jews, but they lost their stature through various sins until the family of Avraham Avinu came and regained the stature. This is discussed in classic works like Derech Hashem. I'm not going into this since the OP asked about Noahide vs. conversion to Judaism, which only is relevant from Har Sinai onwards.)

Regarding 2) and 3) I think the initial assumption is incorrect. The OP implies that the "moral contamination" was abated by accepting the Noahide laws, and that it no longer exists. I don't think this assumption is correct.

The Noahide laws address themselves to the basic moral code. The Torah relates to being "holy". The "moral contamination" is related to the concept of holiness i.e. spiritual sensitivity to sanctity (kedusha). This level of sensitivity only exists in the full sense by Jews. Ex: if a non-Jew eats a cheeseburger nothing negative happens. If a Jew eats a cheeseburger he has done a terrible sin and has done spiritual damage. Eating a cheeseburger isn't a "moral" crime, it's a violation of the laws of sanctity.

In this sense there is still the "moral contamination" i.e. the lack of spiritual sensitivity to the sanctity of the commandments. As mentioned in the answer to 1) part of the covenant between the Jews and G-d is that the Jews will be a "sanctified nation." It's this sanctity which is being referenced.

Answer 2

The Rambam writes in the laws of kings, that at Mount Sinai Moshe Rabeinu recieved the direct commands from G-d, the Creator of the universe, at mount sinai, to encourage all nations (descendents of Noach) to accept upon themselves the 7 mitzvos that Noach was earlier commanded

Conversation, on the other hand, has always been optional and not necessarily encouraged, as the talmud says (don't remember exact source) "converts are difficult for the people of Yisroel"

So to answer the question:

"By what authority, and roughly when, did Judaism concede that adoption of Noachide laws was an acceptable alternative to conversion?"

By what authority? By the authority of the Creator of the universe

Roughly when? As of now, 5781 from the creation of the universe, roughly 3333 years ago, did Judaism concede that adoption of Noachide laws are an acceptable alternative to conversion

Ah, but why haven't the earlier generations been encouraging them onto the masses in the same way as now?

Because it was dangerous at the time, the earlier nations used to be a lot more hostile and dangerous towards the Jews, especially if they were being convinced to change their lifestyles etc

This is the only time pretty much that the people of Yisroel can safely interact with non jews and explain to them what they must do etc. without being in the same kind of hostile environment