How exactly does He do that? Because He cites a few sayings from what WE NOW recognize as Scriptures? Does He take aside the Sadducees and tell them that they are wrong to accept ONLY the Torah? Does Jesus lay out the entire Canon? Does Jesus quote from EVERY book of the Protocannonicals? You are being anachronistic to think that Judaism was some monolithic organization where everyone already agreed on what WAS Scripture. The Essenes had their own ideas - and the Pharisees did as well. Apparently, there wasn't a demand to lay out the Canon until after the Fall of Jerusalem.
The apostles never cited the septuagint as authentic.
It is telling that they considered it the authentic word of God when they cite FROM it, CALLING it the Word of God. My, how you change your standards in the same paragraph. Matthew cites from the Septuagint Isaiah, calling it Scriptures, and it is not Scriptures? WOW.
The allusions are of no more weight than their citing pagan poets and philosophers.
Allusions? How about word-for-word citations? When Matthew records the Septuagint version of Is 7:14, rather than the Hebrew version, calling it the Word of God, you totally disregard that? Please.
Regards
"How exactly does He do that? Because He cites a few sayings from what WE NOW recognize as Scriptures? Does He take aside the Sadducees and tell them that they are wrong to accept ONLY the Torah? Does Jesus lay out the entire Canon?"
Actually He does. In speaking to the religious leaders who would be using the Hebrew Canon, He says, (John 5:39), "Search the scriptures; for in them ye think ye have eternal life: and they are they which testify of me."
As Josephus, the renegade, traitor Jew, sycophant of the Emperors Flavius and Titus wrote soon after Christ, Josephus said "Although so great a time interval has now passed, not a soul has ventured to add or remove or to alter a syllable, and it is the instinct of every Jew, from the day of his birth, to consider these Scriptures as the teaching of God, to abide by them, and, if need be, to cheerfully lay down his life in their behalf." His enumeration and description of these books show that they were the same as those of the Old Testament as we now have it.
There has always been tension in the early church between the Greek speaking Christians and the septuagint and the Jewish christians and the Hebrew Canon. The Gentile Christians as can be expected used the septuagint since it was in their language and Rome, in moving to solidify its political and religious power, used it to remove the "jewishness" of the church.