Let me just weigh in on a couple of things related to this discussion.
- I do think the dating of second Isaiah is a serious issue for the Book of Mormon. Mormons have historically argued for the unity of Isaiah. I think a good representation of that position is given by John W. Welch, "Authorship of the Book of Isaiah in Light of the Book of Mormon," in Donald W. Parry and John W. Welch, eds., Isaiah in the Book of Mormon (Provo: FARMS, 1998), 423-37. On the other hand I tend to accept the scholarly mainstream on the issue. The evidence seems to support both a Deutero and Trito Isaiah, and arguing for a Deutero Isaiah before 540 BCE is a bit difficult.
- I don't think that the existence of 2nd Isaiah necessarily implies that the Book of Mormon isn't an ancient document: (a) The first solution compatible with ancient origins is to argue for an early date of composition of 2nd Isaiah. (b) The second is to argue like tooblue for anonymous redaction/editorial activity at later date by Mormon or somebody else. I think the biggest problem with this is that it doesn't seem to be consistent with Mormon's later non-anonymous redaction/editorial work. (c) Following Blake Ostler's lead argue that the Book of Mormon is "a modern expansion of an ancient source." Given the "midrashic" nature of the Inspired Version of the Bible (JST) a "modern expansion of an ancient source" seems reasonably consistent at least to me with Joseph Smith's revelatory process. Still this view is not without its notable detractors.