The book was written by many ancient prophets by the spirit of prophecy and revelation. “Their words, written on gold plates, were quoted and abridged by a prophet-historian named Mormon.
https://www.churchofjesuschrist.org/stu ... ang=eng#p2
So the content of The Book of Mormon should, 100%, have been written no later than the 1st Century, ending with Mormon’s contribution. If it contains anything that dates after that, then it isn’t what it claims to be. In the 17th Century some people who were putting together the KJV Bible entered some incorrect content into it. They made mistakes with the text. We know that now, but it was not known at the time they made them. Those same mistakes made by people in the 17th Century appear, verbatim, in The Book of Mormon.
However they got into The Book of Mormon is up for discussion. The most obvious answer is that someone plagiarised the KJV Bible when they were putting together The Book of Mormon manuscript. What isn’t up for debate is that the Church statement about what the Book of Mormon is (see above) isn’t correct. What’s worse is that the Church knows it’s not correct. Yet they maintain and promote that incorrect narrative. They are lying. Joseph Smith lied about the contents of The Book of Mormon when he wrote the introduction to it.
But I have a proposal. The Church can amend their fraudulent claim about what the Book of Mormon is. I’ll even write it for them. “The book was written by many ancient prophets by the spirit of prophecy and revelation, and by plagiarising some of the KJV Bible.”
Alternatively the Church could remove the plagiarised content entirely. What it should not do is maintain its current dishonest position.