600-Year Prophecy

Why wasn't King Benjamin aware of Nephi's 600-year prophecy of the coming of Christ?  The 600-year prophecy of Christ's coming was recorded in the small plates of Nephi and passed down via Jacob's line and not the kingly line which kept the more historical large plates of Nephi.  The small plates were eventually given to King Benjamin but if they had been written in Egyptian script, it is uncertain if Benjamin could have read them. It wasnt until Mosiah 28, that King Mosiah uses the seer stones to translate the Small Plates of Nephi and the Jaredites Plates.

Mark in the Book of Mormon?

Some scholars believe that the last part of Mark was added at a later date because the earliest manuscripts dont have this small portion. A particular saying in this possible added portion is found in the Book of Mormon and the Doctrine and Covenants talking about "signs following those who believe".  

Despite personal experience that this divine promise is absolutely true,  we shouldn't forget the likely explanation that the added text in question very likely came from the early unavailable source known as "Q" or the "Sayings of Jesus".   Q is the likelt source for the added material in Mark and as an authentic source of the words of Jesus, we shouldn't find it a surprise to find Jesus sharing His same words in other scripture. 

Also, the main reason why scholars claim multiple authorship of Isaiah is because many don't believe a prophet can speak to things beyond their own time period (eg Cyrus).  The literary analysis is divided but the best studies overwhelmingly demonstrate unity.