Actually, the use of the Septuagint was quite common even in Palestine in the first century AD. However, the Epistles were written for the Gentiles and hellenized Jews of Asia Minor, and the Gospels followed when the Church was already in exile, and were intended for the Gentile converts, so using a Hebrew source was pointless.
the later formulators of the Greek OT made it conform with the New Testament Text, and thus the Septuagint that you have in your hand quotes from the New Testament, not vice versa
If that is so indeed, then a vast number of references made by the Apostles to the OT become meaningless, since they differ in meaning and content from those in the Hebrew Bible.
The same would hold true of Matt 1:23, where Christian Bibles specifically use the word "virgin" and not "a young woman."1
Is that not an example where all Christians "adjust" the New Testament to fit the Old?
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1The Hebrew word for young woman Isaiah used is 'almah, and the Greek word is korasion. The Hebrew word for virgin is bethuwlah, and the Greek word parthenos. The Septuagint uses parthenos in place of 'almah.
The same is true of many other words we use, starting with the word Messiah. The meaning of the Hebrew word meshiyah is unlike anything we assign to it. Other phrases commonly encountered such as the "world to come" have been "retro-engineered" by Christian founders to mean something they never meant in Judaism.
Frankly, accusing the Orthodox of retro-fitting the OT to agree with the NT is like the pot calling the kettle black, since all of Christianity is based on finding the "foreshadowing" of Christ in the OT which the Jews deny as much as Christians deny LDS to be a 'fuller' revelation of Christ.
All of Christianity is 'retro-fitting' Judaism to fit the NT, imo, beginning with abandoning such practices as circumcision and dietary laws for the Gentiles. Christ never taught or even hinted at that. All that was later added ex post facto in Acts.
As for various versions of the Septuagint floating around, they were not Septuagint but Greek translations of the OT written by a convert to Judaism and an Ebonite apostate, masquerading as 'Septuagint.' The only one that was written by a Christian was a 3rd century translation from Ephesus. The Christians there did not ask for the Library of Congress verification of various books. People read what was available. If it was in Greek and it looked like the OT they assumed it was the Septuagint.
There were no copy-right laws. Among Christians, the people copied whatever they felt like copying and in any way they wanted. If the scribe felt the copy he was copying was flawed, he would insert a 'correction' of his own. We are not even sure of the authorship of some of the NT books, let alone of the veracity of the flood of stuff that came along centuries after.
Eventually, Christian texts were made more flawless and more seamlessly connected. The Codex Alexandrinus is a perfect example of such efforts, which coincided with the more elaborate theological definitions and practices of the Church. The resultant Christian canon by the end of the 4th century AD was a carefully screened and redacted set of books, with all the originals "conveniently" lost.
If that is so indeed, then a vast number of references made by the Apostles to the OT become meaningless, since they differ in meaning and content from those in the Hebrew Bible.
That is not so. Jesus didn't hand each of the apostles the exact same copy of the 22 books of the OT for their quotations --- the exact same copy incidentally that then disappeared for centuries, only to surface magically when the Septuagint was being recreated and revised and reconstructed ad infinitum.
The NT writings are called the "New" Testament not the "Old" Testament. The New Testament had new things in it as well as old things. Jesus' teachings encompassed old things as well as new things. So why shouldn't the Holy Spirit through the writers of the NT say some of the old things in new ways for the New Testament Church? Isn't the God who gave the Jews the OT free to paraphrase His word if and when he chooses as the application requires to elucidate and further its meaning for a now wider audience. And some of those could be copyist errors, like Acts 15:17.
As it turned out, many Jews were criticizing the new faith of merely copying the Jews' scriptures to create its documentation. If the writers had just sat down and copied everything out of the book of the Jews, this criticism would have been justified. But the New Testament was written under the inspiration of the Holy Spirit who would help them to remember what Jesus had said and inspire anew.
What I think is more evidence of the inspiration of the Hebrew OT and the Greek NT is that they were transmitted separately, by opposing parties with separate agendas often hostile to one another, with challenges and accusations going back and forth. And yet after centuries of transmission under conditions in which each used its scriptures to challenge the authenticity of the others', the two texts emerge centuries later from separate hands with still so many Hebrew OT passages conforming to their NT Greek equivalent. God looked after his word in the hands of the Jews just as he did in the hands of the Greeks, and those translations faithful to it.
The same would hold true of Matt 1:23, where Christian Bibles specifically use the word "virgin" and not "a young woman." Is that not an example where all Christians "adjust" the New Testament to fit the Old?
Nope. When the Hebrew text refers to a damsel who may or may not be a virgin, the Hebrew word "naarah" is used. But the Hebrew "almah" is used in Isaiah 7:14 with the same meaning as it is used in Genesis 24:43 as regards Rebecca who was a "virgin". And it describes the damsels [almah] that played the timbrels before the congregation of Israel [Psalm 68:25] and these damsels all had to be virgins, young unmarried girls who had not known a man. Everywhere that "almah" is used in the OT it means a "virgin" --- a young girl who has not known a man. Thus the Greek "parthenos" is the equivalent of the Hebrew "almah".