I do not intend this to be the answer, merely an element that I think helps pave the way for a clearer understanding of the passage concerning your question, to be held in conjunction with some of what has already been brought forward in previous answers.
I think the difficulty in understanding this passage, stems from a misunderstanding on how prophecy unfolds.
God instructed Israel about how to know if a prophet spoke from Him (Deuteronomy 18:15-22):
15A prophet from among you, from your brothers, like me, the Lord, your God will set up for you you shall hearken to him.
טונָבִיא מִקִּרְבְּךָ מֵאַחֶיךָ כָּמֹנִי יָקִים לְךָ יְהֹוָה אֱלֹהֶיךָ אֵלָיו תִּשְׁמָעוּן:
16According to all that you asked of the Lord, your God, in Horeb, on the day of the assembly, saying, "Let me not continue to hear the voice of the Lord, my God, and let me no longer see this great fire, so that I will not die."
טזכְּכֹל אֲשֶׁר שָׁאַלְתָּ מֵעִם יְהֹוָה אֱלֹהֶיךָ בְּחֹרֵב בְּיוֹם הַקָּהָל לֵאמֹר לֹא אֹסֵף לִשְׁמֹעַ אֶת קוֹל יְהֹוָה אֱלֹהָי וְאֶת הָאֵשׁ הַגְּדֹלָה הַזֹּאת לֹא אֶרְאֶה עוֹד וְלֹא אָמוּת:
17And the Lord said to me, "They have done well in what they have spoken.
יזוַיֹּאמֶר יְהֹוָה אֵלָי הֵיטִיבוּ אֲשֶׁר דִּבֵּרוּ:
18I will set up a prophet for them from among their brothers like you, and I will put My words into his mouth, and he will speak to them all that I command him.
יחנָבִיא אָקִים לָהֶם מִקֶּרֶב אֲחֵיהֶם כָּמוֹךָ וְנָתַתִּי דְבָרַי בְּפִיו וְדִבֶּר אֲלֵיהֶם אֵת כָּל אֲשֶׁר אֲצַוֶּנּוּ:
19And it will be, that whoever does not hearken to My words that he speaks in My name, I will exact [it] of him.
יטוְהָיָה הָאִישׁ אֲשֶׁר לֹא יִשְׁמַע אֶל דְּבָרַי אֲשֶׁר יְדַבֵּר בִּשְׁמִי אָנֹכִי אֶדְרשׁ מֵעִמּוֹ:
20But the prophet who intentionally speaks a word in My name, which I did not command him to speak, or who speaks in the name of other gods, that prophet shall die.
כאַךְ הַנָּבִיא אֲשֶׁר יָזִיד לְדַבֵּר דָּבָר בִּשְׁמִי אֵת אֲשֶׁר לֹא צִוִּיתִיו לְדַבֵּר וַאֲשֶׁר יְדַבֵּר בְּשֵׁם אֱלֹהִים אֲחֵרִים וּמֵת הַנָּבִיא הַהוּא:
21Now if you say to yourself, "How will we know the word that the Lord did not speak?"
כאוְכִי תֹאמַר בִּלְבָבֶךָ אֵיכָה נֵדַע אֶת הַדָּבָר אֲשֶׁר לֹא דִבְּרוֹ יְהֹוָה:
22If the prophet speaks in the name of the Lord, and the thing does not occur and does not come about, that is the thing the Lord did not speak. The prophet has spoken it wantonly; you shall not be afraid of him.
כבאֲשֶׁר יְדַבֵּר הַנָּבִיא בְּשֵׁם יְהֹוָה וְלֹא יִהְיֶה הַדָּבָר וְלֹא יָבֹא הוּא הַדָּבָר אֲשֶׁר לֹא דִבְּרוֹ יְהֹוָה בְּזָדוֹן דִּבְּרוֹ הַנָּבִיא לֹא תָגוּר מִמֶּנּוּ:
But this begs the question, what of those prophecies that speak into the future beyond what we can see if it will come true. Are we to believe that prophet?
Or, if the purpose of that prophecy were to bring comfort or hope, how could the people derive comfort or hope from a message they were not sure was from God?
We know that the Assyrians did in fact invade and seize Jerusalem. And, from an historical hind sight view, we conclude that this child, Emmanuel, was born in the era this was prophesied, and that, before he came to discern between good and evil, the Assyrians invaded. Otherwise, Isaiah would have been executed as a false prophet rather than esteemed a true prophet as he is considered to this day.
Thus, it is helpful to see this passage as a sign of the prophet affirming the rest of his prophecy.
Rashi expounds on the unfolding of the events of Isaiah historically, if you would like to study that. and the answer above gives some helpful insights into the history of it all.
One more thing, it is obvious that the child named Emmanuel was not himself "God with us." But that His name exalted "God with us." Hebrew children were often named after the activity or attributes of God. Consider even the names of the prophets. The names were always a testimony and a reminder of God in their lives.