The correct phase in this case.
Not specific enough as it doesn't take into account that where the context is unambiguous, the word almah always refers to an unmarried young woman or adolescent girl.
Because there is no reason to infer that in the context of Isaiah 7. The prophecy is not about the woman who is pregnant, it's about the enemies of Ahaz and Judah.
We infer it from the other usages, since here it is ambiguous. The woman and the name Immanuel are significant. She is prompted to call her son that because while he is still an infant, living in poverty because of the invasion, a deliverance happens.
Matthew interprets this as a foreshadowing of the birth of the one who would save his people from their sins. If only God can do this, then in the person of Jesus, God is with us in the fullest possible sense.