I know that the Septuagint was written 1000 years before the Masoretic text and that when Jesus took the scroll of Isaiah in the synagogue at Capernaum and read it before the Jews there and told them this scripture was fulfilled in their hearing. He read it from the LXX that was readily available to all the Jews because they did not have the original Hebrew text at that time. It was by God's providence that the LXX would the text of God's Word, thus preserving His scriptures.