Isaiah and the Book of Mormon

Deutero-Isaiah (or is this one Trito?) writing about Jesus. 

If the fact that Isaiah was written by more than one person isn't enough to bring on at least a little doubt, consider this fact: anything from Isaiah chapters 40-66 that made it into The Book of Mormon either verbatim, slightly altered, or paraphrased (1 Nephi 10:8, 13:37, 17:36, 20-21, 22:8 & 10-11; 2 Nephi 6:6-7 & 16-18, 7-8, 9:50-51, 10:9, 26:25; Mosiah 12:21-24, 14, 15:10-11 & 14-18 & 29-31; 3 Nephi 16:18-20, 21:8-10 & 29, 22; Moroni 10:31) was written after Lehi & Co. left Jerusalem. So how the hell did they make it onto the brass plates and over to the New World?

You could doubt the authorship of The Book of Mormon at this point if you wanted to. Or you could simply doubt the scholarly understanding that Isaiah was written by multiple authors on into the years following Lehi's departure. After all, we know that The Book of Mormon is true and that "the wisdom of this world is foolishness" (1 Corinthians 3:19).

Problem solved! Except there's still the whole plagiarism thing you have to consider too...

The biblical authors

Mormons "believe the Bible to be the word of God" (Article of Faith 1:8). Does this go for all of the words in all of the books? I ask because Bible scholars have found that, for example, Moses didn't write the first books of the Old Testament, Solomon didn't write the Psalms, Isaiah was written by at least two people, and Paul is not the author of all of his epistles. Those are significant problems, but there are still more.

Are they still the words of God if we know that the author is different than what was traditionally believed? Do we need to treat the questionable books, chapters, and verses with any particular care? How can the Bible's pseudopigrapha not be subjected to doubt? Do we need to revise the eighth Article of Faith? Couldn't one of our prophets, seers, and revelators go through and separate the actual words of the prophets and apostles from those of the impostors? What would remain of the Bible were we to remove everything of false or highly questionable authorship? Would we finally have a bulletproof Bible full entirely and only with the Word of God?