The BofM was heavily edited and pulled from the records of the Nephites. It was never meant as a comprehensive guide on the culture, language, religious rites, etc of the Nephite people. The aim of the Book of Mormon is to clearly get people to speak directly with God. I believe the events around the Book of Mormon was contrived by God for just this purpose.
You'd have to convince me first that there is such a thing as Nephites.
The Bible suffers the same historical problems. There just is no evidence of a historical Adam and Eve, Moses, great flood, Abraham, and so on. People who claim the Bible is historical and the BofM isn't are just being hypocritical. A book of fiction set in historical situations doesn't make that book non-fiction.
Myth is a genre of book in the Bible. Meaning, stories that convey truth but aren't to be taken literally. The five books of the Pentateuch are myth. It's only Mormons, Evangelicals and some atheists that insist these books are literal histories.
I've never heard the Book of Mormon described as anything by Joseph Smith, it's author (translator if you must in order maintain a semblance of dialogue), as history. I've never seen the LDS church teach that the Book of Mormon books are not to be taken literally.
The Book of Mormon is as authentic as the Bible. Its claim is God will tell you that it is from him. That is easy to verify in fact. All you have to do is ask and if God doesn't show up and tell you it is true, it isn't. The same can be said of the Bible. Without a real God to back it up, it is just fiction.
Mormons are the only people who I know of that think God is going to show up when you pray about a book. I have never met a Catholic or mainline Protestant who has prayed, asking if the Bible is true. Catholics, in particular, are not sola scripture.