Brant A. Gardner addresses Isaiah authorship in the Book of Mormon.
Brant A. Gardner, Second Witness: Analytical and Contextual Commentary on the Book of Mormon, 6 vols. (Salt Lake City: Kofford Books, 2007), 1:384
If our understanding of textual development allows the possibility of accepting that another writer added entire sections to Isaiah, we may just as easily accept that another writer edited Isaiah from his historical perspective to mark a fulfilled prophecy. In this way, we may continue to accept the complexities of textual transmission, understand the statistical and stylistic reasons for seeing the book of Isaiah as the work of a single author, and accept the testimony of the Book of Mormon as confirmation that those chapters existed when Lehi left Jerusalem with the brass plates. As noted above, this does not explain the Book of Mormon Isaiah passages without at least the lingering issue of translation. The passages appear in the post-redaction state (if that is what actually happened) rather than the pre-redaction state in which they would have been available to Lehi. This is an issue for the discussion on the translation process.