Well, that is precisely the point. Who were the mainstream Jews, was Judaism monolithic at the time of CHrist. The answer is no. So when you say the Jewish OT was settled and universally agreed by all Jews in the Roman Empire at the time of Christ. That is not correct. The Jews in Qumran were likely the Essenes, who Josephus wrote about, and it is plausible that Saint John the Baptist was influenced by them. Clearly the Jewish Zealots had a different understanding of the Jewish OT than the Essenes and a different one from the Pharisees and the Sadducces, who had a totally different understanding of Canonicity of the OT themselves.
So when the FR Protestants where say the Jewish OT of 39 books, that really is the tradition of the Rabbinical tradition which comes from the Pharisees, who were the only group that survived the Roman Armies destruction of the Temple and war that took place around 70AD. Many of the Greek Hellenistic Jews in diaspora, who used the LXX, became Christian.
That touches on the real issue. If the authority of Judaism was passed to the Church instead of to the rabbis, then it would seem that one must accept Mariology and the Papacy and other accoutrements of Catholicism. There is no coherent religion without some kind of authoritative tradition, which is why intellectuals who become believing Christians tend to choose Catholicism.
A small angle of error at a close target becomes much more obvious at longer distances.