Three problems:
Jews have historically carried on hypothetical arguments long after final decisions were accepted. This still happens today.
The Dead Sea Scrolls prove nothing, in that many of them may have been seen to be heretical texts containing the written Hebrew name of God. Their burial or hiding may have been to keep them out of public hands.
The term ‘canon’ doesn’t really mean the same thing to Jews as it does Christians. The Five Books of Moses with the explanations received at the same time have absolute primacy. That’s the opposite of the Christian view, which views all scripture through the New Testament.
And you have provided in that paragraph an argument for the view of traditional Christians, Orthodox, Latin, monophysite, or Nestorian, that the judgement of the Church as to what constitutes the canon of Scripture to which the Christians should attend supercedes the judgement of rabbis who deny both the fulfillment of the Old Covenant prophecies by Our Lord Jesus Christ and His divinity.
I had left out the Nestorians before because the history of their approach to the canon is more complex -- East Syriac Old Testaments were translated mostly from the Hebrew (though not from the Masorete) rather than the LXX, but portions were translated from the LXX, they sometimes have all the books found in the LXX but not the Masorete, sometimes have some of them, and sometimes have others that Orthodox Bibles either omit or relegate to an appendix (like 3rd and 4th Maccabees) plus a text that exists only in Syriac -- the 2nd Baruch or the Apocalypse of Baruch).