The evidence suggests that Joseph Smith was not familiar with the content of the Book of Mormon while dictating.
One example of how he was not familiar with the content of the book, is how Joseph was unsure how to pronounce many of the names.
His wife Emma said:
When my husband was translating the Book of Mormon, I wrote a part of it, as he dictated each sentence, word for word, and when he came to proper names he could not pronounce, or long words, he spelled them out… Even the word Sarah he could not pronounce at first, but had to spell it, and I would pronounce it for him– Edmund C. Briggs, “A Visit to Nauvoo in 1856,” Journal of History 9 (October 1916): 454
Hiram Page also made a similar observation about the pronunciation of Nephi:
As to the Book of Mormon, it would be doing injustice to myself and to the work of God of the last days, to say that I could know a thing to be true in 1830, and know the same thing to be false in 1847. To say my mind was so treacherous that I had forgotten what I saw. To say that a man of Joseph’s ability, who at that time did not know how to pronounce the word Nephi, could write a book of six hundred pages, as correct as the Book of Mormon, without supernatural power.– Hiram Page, letter to William E. McLellin, 30 May 1847, Ensign of Liberty 1 (January 1848): 63.
Why would Joseph Smith not know how to pronounce names if he had chosen them? Why would he have to spell the names out in syllables, leaving the scribe to put them together?