The fact is that negroes used to be banned from the priesthood on the spurious grounds that their skin colour was a sign of descent from Cain.
I have in fact read most of the Book of Mormon but the poor imitation of Jacobean English was very wearing. I would have thought that a divinely inspired translation could have managed rather better.
And it's not the case that there have been hundreds - or even one dig - in the Americas confirming the Book of Mormon.
And yes it is quite apparent from the Book of Mormon that the text in it's current form dated from after the time the "Golden Plates" were supposedly discovered. The American nationalism itself dates it to the late 18th Century at the earliest.