We would say that the Septuagint is an inspired work, a new step in revelation from the Hebrew in preparation for the Gospel. That does not change the fact that the Jews tampered with the prophecy in response to Christianity.
The trouble is that it kind of feels like you are making it up as you go along now.
Anyway, if you read it in context, Isaiah 7:14 isn't a prophecy about the Messiah, it's part of a prophecy about the fate of Israel and her enemies during the time of the king Ahaz (also a messiah).