Interesting info, Barry, and great research with the LXX - that's going in my files.
However, I don't think anyone is saying that the prophecy MUST be about Ahaz's wife and Hezekiah. I've read ideas that maybe it was a woman espoused to Isaiah himself, another maiden in the room (a servant or something), or someone else.
Note that the purpose of the prophecy, contextually, is to reassure Ahaz that the enemy kings conspiring against him would not be successful. Speaking about the prophesied child, Isaiah says "For before the child shall know to refuse the evil, and choose the good, the land that thou abhorrest shall be forsaken of both her kings." (verse 16) The opposing country was forsaken for both those opposing kings WAY before Jesus was born.
Also, verse 14 says the child's name was "Immanuel". Christ's name was "Jesus", as given by the angel to Mary.
I believe that in Ahaz's day, there was a child born, named "Immanuel", and that before that child was very old, the opposing kings were eliminated. It seems to me that I have to believe that, or else I have to believe that the prophecy was a lie to Ahaz.
Furthermore, the inclusion of the definite article rendering it THE Virgin, must also point beyond the local situation.
Actually, I really don't think so. Context. If I'm in my garage with my brother, and there is only one car present, and I say "THE car", I mean the car that is present and I certainly don't mean to imply that in all human history, there has only one that qualifies as being called THE car.
For a more Biblical example, Rebekah is called "THE virgin" in Gen 24:43 (which was also a prophecy, no less). Deut 32:25 (also a prophecy) talks about God destroying "THE young man and THE virgin, THE suckling also with THE man of gray hairs" with the sword. Even Israel is called "THE virgin" in several passages.
Don't misunderstand me, I do see some messianic content in the passage, but more of a "type" than a primary reference. God know the end from the beginning, and I believe he gave the Immanuel prophecy to Isaiah knowing (intending) Matthew would find "fulfillment" of that in Jesus - similar to Hos 11:1/Matt 2:15 like I mentioned earlier.