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".
Ping to 11,228.
This is somewhat inconsistent considering the "newly married" part, as it is almost certain that a newly married woman would no longer be a virgin.
The difference is that in 'almah her presumed virginity is just that — presumed. In the bethuwlah it is a certainty.
In the former, also, it is not the object; in the latter it is. From a strictly Christian point of view, it is critical that virginity is a certainty not only before marriage but after the conception.
Thus, when the Septuagint refers to the woman as a virgin, it places the virginity in focus and makes a statement of fact, not a presumption. The Hebrew text does not assure us of virginity, but simply presuposes it.
Do we have the original Hebrew OT? Do we have anything regarding the written Bible older than 500 BC? God did not dictate, word-by-word, the Scripture.
Absence proves nothing except absence itself.