Interesting choice of an authority you select there. Why would a Jewish council, reacting to the rise of Christianity and meeting after you yourself admit the "Jewish Holy Spirit" was no longer around to guide them, be an authority figure for what the Christian Bible should be?
Did Jesus breathe the Paraclete upon the Church or this council at Jamnia?
SD
And what would establish "The early Christian canon", precisely?
Not the opinion of the post-temple Jews. That there was a Jewish "council" of Jamnia at all was precisely because there had been a fatal change in Judaism with the destruction of the Temple and the loss of the center. The struggle was to preserve what could be preserved, and the Palestinian Jews were much more ethnically focused, and also much more polemically and violently anti-Christian, than Alexandrine Jews or the Greek-speaking Jews of the diaspora. There is no good reason to give the highest authority in determining the Jewish canon to those bitter-ender Palestinian Jews who hated and persecuted the Christians most.
By contrast, the Septuagint itself was a translation made by translators sent under the commission of the High Priest of the Jewish Temple in an era BEFORE the Christian polemics were occurring, with Christians taking the Greek Septuagint, and bitter-ender ethnic Jews in Palestine rejecting anything written in Greek.
The Council of Jamnia, whatever it was, was composed of and represented people who were committed, many of them murderously so, anti-Christians. That's singularly bad authority on which to base any canon at all.
The choice of the High Priest and his translators in an earlier age, which produced the Greek Septuagint that went into universal use is more authoritative: the High Priesthood was still THE established leader of God's One True Faith on earth at the time. The Jews of Jamnia were already deeply in error, had already rejected the Messiah, and were no longer God's One True Faith. The Christians were, at that point.
When the Septuagint was translated, by contrast, the High Priest who ordered it DID have the proper authority to do so.
The Septuagint has greater authority than the Massoretic Text for that reason.
But let's leave aside what the Jews thought. What early Christians thought is far more important.
Go through the earliest Christian Councils that attempted to determine canonical works. You will discover that every single one of them included at least some of the Deuterocanonica, along with books that do not appear in the current New Testament. And they all left out some books now there.
So, Christians disputed the boundaries of the Old Testament, and the whole of the New Testament, for a long time. The Bible first assumed its current shape with Pope Damascus in the late 300s, and Jerome. Jerome had an opinion, and was quite pro-Judaic in his preferences of texts, but he was also doing his work more than 300 years after the fact. There is no particular reason to give his opinion of what ought and ought not be canonical heavy weight, considering that not one single Church Council that dealt with the OT canon agreed with him. Jerome didn't claim special revelation from God on that score either.
He had an opinion. Councils before him didn't agree. And the Pope of his time didn't either, which is why the Vulgate contains the works that Jerome didn't want in there.
Dig around in those early councils. Deuterocanonical works are there in those lists.
More interestingly, and demonstrating that Catholics don't have the vaguely idolatrous fixation with the Bible as the Be-All and End-All of revelation, is the fact that the Eastern Rites of the Catholic Church share the Orthodox Canon, which includes 3 and 4 Maccabbees. The Western Rite does not. And this difference in the Bibles of the Catholic Church is NOT a basis for Schism.
Because God the Holy Spirit is in the CHURCH, and the Church is what interprets the Book and the times and everything else. Just like the High Priest and Sanhedrin before them. The Samaritans tried their damndest to assert that the TORAH was the Law, and that the Torah superseded the authority of the High Priest and the Temple, rendering the latter unneccessary. The Torah was sufficient for all.
The rest of the Jews didn't consider the Samaritans Jews at all, on the grounds that, actually, authority did not repose in the Torah. It reposed in the only men who had been given the authority by God to finally interpret the Torah, and they were the High Priest and the Sanhedrin, then.
Today, they are called the Pope and the Vatican Curia.
With the death of Jesus, the High Priest and Sanhedrin ceased to hold any authority at all. They became ethnarchs and guardians of a tradition. The High Priesthood devolved on Peter and his successors, and the authority of the Sanhedrin devolved upon the Sanhedrin and theirs.
Jamnia had no authority to decide anything about the Bible, any more than Mohammed and his clan did. Because in 90 AD, salvation was no longer of the Jews, and the Jewish clergy ceased to have any authority from God at all.