I believe that this text is written for the time in which it occurs. Using the notes in The NOAB 4 translation of the word woman in Isa.7:14 is translated as virgin in the Septuagint, the New International Bible note indicates that the Hebrew almah "young woman" refers to queen in a Canaanite parallel. That then might mean Queen Abi (King Ahaz's wife) or Abijah (.2 Kings 18:2; 2 Chr 29:1). Young woman may also refer to Isaiah's wife whose son is named Immanuel.
Isa. 7 is also a great deal of metaphor. Isaiah's son is with Isaiah and his name means "a remnant shall return" which should bolster Ahaz's skepticism. God through Israel reveals the alliances between Damascus and Samaria in vs 8-9. Immanuel means "God with us" but in light of the actions that Ahaz is considering, it is also symbolic of God's faithfulness to Ahaz despite his wandering away from God.
While Collins writes that vs. 18-24 are the oracle, I think the verses belong with the prophecy in vs.13-25. Collins also writes, chapters 7 & 8 are grouped together with a common theme of judgement and mercy.