Isaiah 7 & 53: should prophecies be prooftexts?

My great-grandfather Luther Weigle [pictured here] was the dean of Yale Divinity School and chair of the translation committee for the original RSV Bible. He incurred the fury of the fundamentalists when he chose to translate the Hebrew word almah in Isaiah 7:14 as “young woman” instead of “virgin.” They actually burned RSV Bibles and sent the ashes to him in the mail. The reason? Isaiah 7:14 is referenced by Matthew’s gospel as an explanation for Jesus’ virgin birth. But Isaiah 7:14 also refers to the “young woman” who was Isaiah’s prophetess wife and definitely not a virgin. In Isaiah 7 and 8, she bore Isaiah two children with prophetic names related to their immediate historical context. Does the doctrine of Christ’s virgin birth depend on translating almah as “virgin” in Isaiah 7:14? Only if Isaiah 7:14 is expected to function as a prooftext for that doctrine, which raises a larger question: to what degree should Old Testament prophecy be used as prooftexts? And if Isaiah 7 is allowed to have less than a perfectly mapped correspondence to the circumstances of Jesus’ birth, then can we apply the same hermeneutical boundaries to the relationship between Isaiah 53 and the circumstances of Jesus’ death on the cross?