We need to understand what the NT writers were doing with the OT.
No. That's a side issue, what is relevant is how the text should be interpreted.
Sometimes Christians will talk about the hundreds of prophecies that Jesus supposedly “fulfilled,” as if this was something miraculous.
That's their problem. Prophecy is fundamentally connected to the Word of John 1:14.
If you look at Hos 2:6 or Is 7:14 in their original context, they were about something at the time. They weren’t predictions about the 1st Cent. (Is 53 is more difficult.)
It's Hosea 6:2 that's relevant here (my mistake with the reference that you quoted). Isaiah 7:14 is referenced by Matthew 1:22-23, and so it supports the point that the gospels reference earlier prophetic texts for validation.
the citation of Is 7:14 is a way of putting Jesus’ birth — which Matthew already believed — in an OT context.
It's presented an an explicit fulfilment an prophecy. Prophecy can have a near fulfillment and a far fulfillment.
Now all this was done, that it might be fulfilled which was spoken of the Lord by the prophet, saying...
Matthew 1:22
As John is the new Elijah, this is the new child who will be a sign for the nation.
The specifics of the sign relate to a young child called Immanuel who ate butter and honey, and knowing good from evil, rejected the evil within a specific timeframe. Knowledge of good and evil connects to the reason for the expulsion from the garden of Eden and to eternal life. Knowledge is a key part of the prophecies relating to Yahushua, ans he spoke of the promise of eternal life.
That the LXX (which was understood as being inspired) translated “virgin” was an extra bonus, but I think Matthew might have used the passage anyway.
There's a number of doctrinal differences between the LXX and the Septuagint, such differences highlight points of interest for further study.
It remains that there's a significant prophetic connection between the lion of Hosea and the roaring of Psalms 22, with Hosea 6:6 referenced twice in Matthew and the connection to the cry of dereliction.
For I
will be unto Ephraim as a lion, and as a young lion to the house of Judah: I,
even I, will tear and go away; I will take away, and none shall rescue
him.
Hosea 5:14
My El, my El, why hast thou forsaken me? [why art thou so] far from helping me, [and from] the words of my roaring?
Psalms 22:1
The point remains that the only meaningful interpretation of "Then opened he their understanding, that they might understand the scriptures" involves verses which don't support the Pauline interpretation of the crucifixion. Indeed, the connection to the crucifixion of Psalm 22 highlights this problem.
But I [am] a worm, and no man; a reproach of men, and despised of the people.
Psalm 22:6