The Book of Mormon is the most correct Book they never said perfect or infallible.
Pratt was an Apostle and what he said about the Doctrine and Ordinance have been true from day one of the Revelation!
Those copies of manuscripts you speak are of recorders of thousands of years yet it was not until the 1800's before any archeology has come forth.
The Book of Mormon has only been in our presents for a couple hundred years and in the Lord own time will the knowledge so many are murmuring for come forth!