And you people have a problem with the Book of Mormon exactly why then?
Because the Book of Mormon is a fraudulent document written by a con-man. As Buffalo notes, the Hebrews were real people. The Egyptians were real people. The Samaritans and the Assyrians were real people. However the "Nephites" and the "Lamanites" and the "Jaredites" were all the invention of Joseph Smith. There is no such thing as "Jaredites" or "Nephites" or "Lamanites." They never existed. They are the imaginary peoples of an imaginary story imagined by some guy who loved to tell stories. The comparison is not with Hebrews and Egyptians, but with Hobbits and Elves.
For "Mature Christians," it makes no difference whether or not the author of Deutero-Isaiah was the same as the author of the rest of Isaiah, since both were inspired by God and both had an important, canonical, message. It doesn't impact their faith at all, because they know that the content is the most important thing, and authorship in the ancient world was considered very differently from the way we think of it now. For example, there was no such thing as copyright.
However the transparently bogus nature of the Book of Mormon means that it could not be--that it most certainly is not--what it claims to be or what Joseph Smith claimed it to be. Mature Christians are not bothered by the fact that portions of Isaiah were written later than other portions of Isaiah, since the value of the Book of Isaiah does not derive exclusively (or even mainly) from the authority of its composer(s). Mature Christians are not bothered by the fact that most of the Psalms either pre-date or post-date King David, since the value of the Psalms do not derive from the authority of King David.
On the other hand, the Book of Mormon claims to be the record of an ancient American civilization. But this civilization never existed, and whole passages that must necessarily (if the Book of Mormon is to be believed) have come from pre-exile Jews, are known today with absolute 100% certainty to date from after the Exile. This poses no problem for mature Christians, for the reasons outlined above. But it poses huge problems for Mormons, because it is incontrovertible evidence that Joseph Smith was a liar and a fraud and that the Book of Mormon is not what it claims to be.
Since when did adding real people, places, and events into a fictional account start transforming it from fictional account an actual historical account? Isn't ficition still fiction no matter how many real people, places or events is mentioned in passing?
No one here is saying that the Book of Isaiah or Deutero-Isaiah is fictional. Quite the opposite--we know there were two authors because of the facts included in the book, which postdate one of its authors. Nor is anyone here saying that it is fraudulent. There are many ways for faithful Christians to acknowledge that the Book of Isaiah, including Deutero-Isaiah, was divinely inspired, and not fraudulent.