I also see the reason in Brown's commenatry regarding Prov. 30:19 as well, "there would be no miracle in that passage," altough I can see alternative perceptions.
The virginity of almah is a given, but it is neither the focus nor a special favor, it seems, of hers. Mary's viginity, on the other hand, is absolutely the focus and a special favor.
It's like mentioning a 5-year-old girl. Her innocense is presumed and her (non-existent) sexuality is irrelevant.
With other two terms, betuwlah and naarah, it seems, the virginity is asserted and is in context of who the woman is, as regards her sexuality.
Going back to Isa 7:14, in the Hebrew version there is nothing to imply that (1) the Lord was speaking to her (the 'you' is plural), (2) that the conception was immaculate, (3) that she will remain virgin after 'coceiving in the womb.' In the Septuagint, all these are definitely implied and present.
The problem is, if the Septuagint is a retro-written forgery made to fit seamlessly the New Testament, as Paul kahle claims, then we have a problem on our hands since there is nothing in the form of prophecy (in the context of the Hebrew version) to suggest Isaiah was speaking of Christ.
"For the same Spirit who was in the prophets when they spoke these things was also in the seventy men when they translated them, so that assuredly they could also say something else, just as if the prophet himself had said both, because it would be the same Spirit who said both; and could say the same thing differently, so that, although the words were not the same, yet the same meaning should shine forth to those of good understanding; and could omit or add something, so that even by this it might be shown that there was in that work not human bondage, which the translator owed to the words, but rather divine power, which filled and ruled the mind of the translator. [Augustine:City of God;Book XX;Chapter 43]
If that is one's attitude regarding the work of a translator, then it is not far-fetched to believe that Origen, the notoriuous master of forgeries, who was never bound by any text, and owed nothing to its words, would have simply made the words of his fifth column conform to the New Testament manuscripts which we all know he was also inspired to creatively emend as the spirit moved him.
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?