Whether the Council of Jamnia/ Yavneh even existed is debated by scholars; there never was a "Council" of Yavneh IMO. Yavneh was, indeed, the place from which the Pharisees (also called Rabbis) ran the government of Palestine that had been entrusted to them by the Romans. Presumably some of their debates took place there also, possibly including the famous one about which of the hagiographa "render the hands unclean". But this was no "council". It was simply a lawyers' debate about the precise boundaries of a library that had already been established by usage. Note Joshua ben-Sira's grandson's phrase, "the law and the prophets and the other books of our fathers". Josephus too mentions the law, the prophets, and four other books which contain songs and precepts. It isn't certain what they were, but one plausible guess is Psalms, Proverbs, Song of Songs, and Ecclesiastes, these last two being the very ones about which doubts are raised in the Mishnah. So the Aramaic-speaking Jews of Palestine had one sacred library, established by custom, the Greek-speaking Jews of Egypt had another. The Qumran sectarians gave great respect to the Books of Jubilees and Enoch in addition, and seem to have disliked the book of Esther. But (so I understand) fragments of all the books of the Hebrew Scriptures except Esther have been found at Qumran, suggesting that the so-called "Hebrew canon" was already as stable as the Septuagint, and formed the sectarians' starting point. The Rabbinical debates about whether Ecclesiastes and the Song of Songs "soil the hands" may not even have been very seriously intended, though Akiba seems to have taken them seriously. Perhaps the lawyers were, as lawyers will, merely positing hypotheticals or raising iconoclastic questions for the fun of it.
