For instance, does Isaiah 7:14 have the potential for having fulfillment in some initial applicable sense for Ahaz AND completed in Christ (Matt 1:23)?
Well, if Matthew says the prophecy was fulfilled in Christ, no further debate need be had. That isn't to say that Matthew's declaration is necessarily the one intended by Isaiah, which was the threat of war against Jerusalem by the kings of Syria and Israel (14:1-2). But frequently, when Matthew declares a prophecy fulfilled, he is reading it typologically.
Compare, for example, Matt. 2:23: "He went and lived in a city called Nazareth, so that what was spoken by the prophets might be fulfilled, that he would be called a Nazarene." The prophets said no such thing in the Old Testament. Matthew is referring to Judges 13:5:
Behold, you shall conceive and bear a son. No razor shall come upon his head, for the child shall be a Nazirite to God from the womb, and he shall begin to save Israel from the hand of the Philistines.
Now being a Nazar
ene and a Nazir
ite are two entirely different and unrelated things, and there's no etymological connection between them. What Matthew is saying is that Samson is a type of Christ. You can see even a little of that in this one verse, where it says, "he shall begin to save Israel." Jesus is like Samson in that he is the strong man who plunders the enemy and saves his people.
Back to Isaiah. In Isaiah 7, Jerusalem is under threat. Isaiah gives Ahaz a sign: a child who won't even be old enough to know right from wrong before the threat goes away. Move on to chapter 8, where
Isaiah's wife herself has a child, and God promises that Israel and Syria will be laid waste before he's old enough to talk.
The son's symbolic name is "Immanuel."Matthew interprets him, again, as a type of Christ, for whom "God with us" is literally true.