I guess I would incline to the first of the two options you offer. However, I think that are other options, such as that the language of the Masoretic Text of Isaiah is not evidence of the language used by original authors of the various parts of Isaiah. The language of the biblical books in general was subject to widespread updating in scribal transmission, as we demonstrated in detail in Historical Linguistics and Biblical Hebrew.
I guess I would incline to the first of the two options you offer. However, I think that are other options, such as that the language of the Masoretic Text of Isaiah is not evidence of the language used by original authors of the various parts of Isaiah. The language of the biblical books in general was subject to widespread updating in scribal transmission, as we demonstrated in detail in Historical Linguistics and Biblical Hebrew.