So then are you saying that the Doctrine of Perpetual Virginity rests on the Septuagint's translation here? That might explain the doctrinal agenda underlying the propagation of the myths of the Septuagint by those in the church who adhere to the Doctrine of PV.
The dilemma you are left with is two-fold. If the Jews removed "bethuwlah" from Isaiah 7:14 and replaced it with "almah", they would have done it when? after the new faith was growing, right? at the Council of Jamnia or afterwards, right? But they didn't because the DSS turned up a complete Hebrew Book of Isaiah, a copy that dates back to atleast 100 BC, and the word "almah" is in that copy in that verse.
So then the other dilemma that you are left with is whether the Greek word "parthenos" is the proper translation of the Hebrew word "almah" in the Septuagint. If it is not, then the Greek translators erred in their work, and those who are trusting in the Septuagint are trusting in a flawed document. So which is it --- an accurate rendering or a flawed translation?
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.