Are there any births foretold in the book of Isaiah which scholars have held NOT to be about Jesus, the Messiah?
What textual clues have been used to distinguish between passages that predict some other prophet versus Jesus himself?
From Isaiah 7 ESV (emphasis mine):
1-4 In the days of Ahaz the son of Jotham, son of Uzziah, king of Judah, Rezin the king of Syria and Pekah the son of Remaliah the king of Israel came up to Jerusalem to wage war against it, but could not yet mount an attack against it. When the house of David was told, "Syria is in league with Ephraim," the heart of Ahaz and the heart of his people shook as the trees of the forest shake before the wind. And the LORD said to Isaiah, "Go out to meet Ahaz, you and Shear-jashub your son, at the end of the conduit of the upper pool on the highway to the Washer's Field. And say to him, 'Be careful, be quiet, do not fear, and do not let your heart be faint because of these two smoldering stumps of firebrands, at the fierce anger of Rezin and Syria and the son of Remaliah...
10-16 Again the LORD spoke to Ahaz, "Ask a sign of the LORD your God; let it be deep as Sheol or high as heaven." But Ahaz said, "I will not ask, and I will not put the LORD to the test." And he said, "Hear then, O house of David! Is it too little for you to weary men, that you weary my God also? Therefore the Lord himself will give you a sign. Behold, the virgin shall conceive and bear a son, and shall call his name Immanuel. He shall eat curds and honey when he knows how to refuse the evil and choose the good. For before the boy knows how to refuse the evil and choose the good, the land whose two kings you dread will be deserted.
To quote Albert Barnes' Notes on the Bible:
Its general original meaning is not difficult. It is, that in a short time - within the time when a young woman, then a virgin, should conceive and bring forth a child, and that child should grow old enough to distinguish between good and evils - the calamity which Ahaz feared would be entirely removed. The confederacy would be broken up, and the land forsaken by both those kings. The conception and birth of a child - which could be known only by him who knows “all” future events - would be the evidence of such a result. His appropriate “name” would be such as would be a “sign,” or an indication that God was the protector of the nation, or was still with them.
Isaiah 7:14 is well-known as a prophecy of Jesus Christ's birth; within context, we see that it is also prophesying the birth of another. Isaiah tells Ahab that a son will be born of a young woman, and that while this child is still young, these two kings he worries about will no longer be a threat to him. In fact, Assyria defeated both these nations (Syria and Israel), then turned to subjugate Judah (prophesied in verse 17).
The Keil && Delitzsch Commentary on the Old Testament says this of Immanuel:
His eating only thickened milk and honey, at a time when he knew very well what was good and what was not, would arise from the desolation of the whole of the ancient territory of the Davidic kingdom that had preceded the riper years of his youth, when he would certainly have chosen other kinds of food, if they could possibly have been found. Consequently the birth of Immanuel apparently falls between the time then present and the Assyrian calamities, and his earliest childhood appears to run parallel to the Assyrian oppression. In any case, their consequences would be still felt at the time of his riper youth.