I think that there is a question as to whether the three witnesses are reliable or not.
I'm also pretty damn sure that Native Americans are not descended from Egyptians, and that the genetic information that shows they're not is widely available, and does stack up.
If that was not the case, there would be some super-duper famous scientists right now that had managed to prove a key tenant of Mormonism; either Science or Nature would publish that like a shot. They LOVE overturning apple carts: if you have the hard evidence.
In the real world... that hasn't happened, because they're not descended from there, all the evidence shows that Native Americans came from Asia, migrating across the Bering Strait. It's just 50 miles across the ocean there, it's many thousands of miles the other ways.