First, for a Catholic Christian, the Gospels are the direct message from God and the Old Testament is in many parts vague and imprecise , -- albeit inspired -- prefigurements of it. So, in principle, it is possible that the New Testament clarifies and rephrases the Old. This is why it was wrtiten in the first place.
However, to take "almah" in some way that excludes the sense of "virgin" is absurd, because to say, "behold, a young woman will conceive and call the Child Emmanuel" is a meaningless statement. It is old women conceiving that is near miraculous and is perhaps worth prophesying about, not young ones.
I don’t consider the old covenant (Old Testament) either vague or imprecise, nor do I consider the new covenant (New Testament) unclear.
I consider the old covenant to be the “old” covenant and the new covenant to be the “new” covenant.
It makes sense to me what you posted about a young woman conceiving being a meaningless statement as it was supposed to designate a “sign” from God. Good point!
1) sign, signal
a) a distinguishing mark
b) banner
c) remembrance
d) miraculous sign
e) omen
f) warning
2) token, ensign, standard, miracle, proof
Therefore the Lord himself shall give you a sign! I think (agree with you) that He would pick something designed to demonstrate His omnipotence as opposed to an everyday occurence.