As you observe, the DSS prvoe that the rabbis at Jamnia did not change anything (there was no reason to change!).
You do not consider what most scholars assert: that the Septuagint is a variant of Jewish scriptural tradition. Yet I am sure you do recognize that there are koine Greek fragments of OT material that date back to the third century BC onward that differ from the Hebrew version.
Do you have any clue why would there be 3rd century BC OT fragments in koine Greek? Or did your biblical periphery, Paul Kahle 'establish' they were 'forgeries' as well?
You are obviously discounting the possibility that perhaps the Apostles used the Septuagint scrolls precisely because they were prophetic as compared to the Hebrew version.
Perhaps that's why those following the Hebrew version did not recongize Christ, and those who read the Septuagint did.
Could these variants be what Jesus referred to as "traditions that make the word of God of no effect". Variants are like mutations --- they weaken the species. The Jews were zealous for the accuracy of their original scriptures. Variants were were destroyed and buried so as to not be passed on to later generations.
Yet I am sure you do recognize that there are koine Greek fragments of OT material that date back to the third century BC onward that differ from the Hebrew version.
Most of the fragments are from the Pentateuch, and since they follow the Hebrew text very closely, they are evidence that the first five books may very well have been authoritatively translated --- but that's all. Fragments from the other books are said to be poorly translated and loose paraphrases which means that amateurs were making haphazard attempts but failed. Furthermore if these fragments were found in Cave 4, where 40% of the fragments of the DSS came from, they have to be seriously questioned, since that is believed to have been the place where things were discarded not stored. The Hebrew manuscript of Isaiah, for example, was preserved in a jar because it was valuable.
Perhaps that's why those following the Hebrew version did not recongize Christ, and those who read the Septuagint did.
Actually the Hellenistic Jews who would have been the ones reading the Septuagint were just as violent against the new faith as those in Jerusalem who would have adhered to the Hebrew text.
And let's think about it. If you want to find out if something is true, you would go to the source. The Bereans may have checked their Greek copy of the prophets to see if what Paul told them was true, but then being true Bereans, they would have then checked the Greek translation against the Hebrew text to make sure. Isn't that how you would do it?