I think your dates are off on Isaiah. Also, check the discrepancies of the Masoretic text of Matthew 15:8-9 vs. Isaiah 29:13.
Regards
Origen assumed the accuracy of the Hebrew text in his first column of the Hexapla. He assumed it because it was true in his opinion. No one knew those Greek and Hebrew manuscripts like Origen in his day.
Why would someone with his knowledge of manuscripts undertake a work like the Hexapla, a massive scholarly undertaking for which there was no equal, and base it on a faulty assumption. He wouldn't and he didn't.
Clearly Origen examined all the available manuscripts of his day and saw that the Hebrew text was fixed and unchanging and put that text in column one.
The Greek texts of the OT of his day, however, varied widely. If there was a standard Septuagint version of his day, it would have been in one of those columns --- but there was no standard Greek OT, only multiple versions some more accurate in areas than others.
He needed five columns for the Greek texts and could have used three or four more columns as well for the Greek --- but he needed only one column for the Hebrew because there was only one Hebrew text and he believed that he had it and put it there in column one and then used all the others, especially Theodotian's Greek text in column six to create his LXX in column five.
Origen's Hexapla is testimony to the accuracy of the Hebrew text in his day. Jerome recognized it as well. It was just as that Jew Paul said in that Greek text of the letter to the Romans: unto them [the Jews] were committed the oracles of God and they were faithful to transmit them accurately, even books like Isaiah and Daniel wherein the prophecies of the Messiah clearly point to Jesus as their fulfillment.