You wrote:
“Then why did 70 of the best Hebrew scholars translate it as “virgin” in the Septuagint?”
Most likely they believed that was a better translation according to traditional understandings that had developed since the text was originally inspired.
“Why is a the 10th century masoretic text, and a contemporary usage of the word, to be preferred to the septugint which was written by people who spoke Biblical Hebrew everyday and thought the word meant “virgin”?”
Because when you’re using the Masoretic text you use the Masoretic text. The NABRE is not a translation of the Septuagint.
That doesn't matter. The Jews who created the septugine were translating the Hebrew scriptures and THEY thought the word in question meant "virgin" as used in Isaiah 7:14, therefore when they tranaslated the word into Greek they chose the specific word for virgin. This becomes more signigicant when one realizes that in the second century both Origen and Tertullian warned that the Jews were changing the wording of their Hebrew scriptures to water down the prophecies of Christ.