Proverbs 30:19 not only uses almah in a context that makes is not God's special purpose, but also clearly NOT as a virgin.
Thank you also, Kosta, for a very informative and civil discussion. Well, Strong's has almah in this verse, so I have to admit that I can't explain why Barnes said it was only two. It's usually pretty trusty. :) In any event, the use of almah here in Proverbs can be seen as requiring a reference to a virgin. Here is an interpretation from the Jamieson, Fausset, and Brown Commentary :
"Proverbs 30:18-19 There be three (things) ... yea, four which I know not: The way of an eagle in the air, the way of a serpent upon a rock, the way of a ship in the midst of the sea, and the way of a man with a maid - as all these afford no clue to their mode of action. "The way of a man (geber: a mighty or wanton man) with a maid," whom he is trying to seduce, is so subtle that it baffles penetration. Inexperienced females must not rely on their own wisdom and strength of resolution, as securing them from evil, when they place themselves in positions of danger. The depths of nature symbolize the depths of Satan and his agents. The eagle flies upward, without turnings. like other birds, and soars so high that the eye cannot trace his "way." The serpent, though without feet or wings, trails along the rock wheresoever it will, leaving no impression of its way. The ship, notwithstanding its bulk, speedily traverse many miles leaving no track in the water. So the man who is passionately bent on gratifying his lust after a virgin, adopts varied devices which cannot be traced out fully in detail. The movements of all four are light, gliding, without noise, quick, and the mode unknown to us. Thus the sense gives no sanction to the Jews' use of this verse to deny that halmah, in Isa 7:14, means a virgin: if she were not so, there would be no miracle in that passage." (emphasis added)
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.