I can't say I agree with your explanation here. You're denying what the passage says in my view.
This is what it says:
Isaiah 7:10-12
10 Then the Lord spoke again to Ahaz, saying,
11 Ask a sign for yourself from the Lord your God; make it deep as Sheol or high as heaven.
12 But Ahaz said, I will not ask, nor will I test the Lord!
Ahaz is told by Isaiah to ask for a sign from God, but Ahaz refuses to ask.
It goes on:
Isaiah 7:13
13 Then he said, Listen now, O house of David! Is it too slight a thing for you to try the patience of men, that you will try the patience of my God as well?
14 Therefore the Lord Himself will give you a sign: Behold, a virgin will be with child and bear a son, and she will call His name Immanuel.
Isaiah tells us the SIGN is a "virgin being with child"...which was Mary. Therefore the SIGN that was given to "the house of David" was very much future! Mary was miraculously found to be with child...yet she was a virgin. THAT IS THE SIGN! The shame of it, is that much of Israel had gotten so far from God that they didn't realize Mary was the sign, so they missed it. If the people knew that prophecy they would have recognized Jesus to be that fulfillment.
Matthew tells us that this was the fulfillment of that:
22 Now all this took place to fulfill what was spoken by the Lord through the prophet:
23Behold, the virgin shall be with child and shall bear a Son, and they shall call His name Immanuel, which translated means, God with us.
It's pretty explicit there. I have to ask why are you saying it's not?
You said: "Prophetic signs always have a near to the time fulfillment (which is never exact) which points us to the future fulfillment."
That is not always true. There are several prophecies given that took literally years before the sign of that prophecy came about.