Blam, well aware of the Zuni situation as well as that of the Melungeons. Regarding the Melungeons, recent DNA studies revealed they have some "black" ancestry, but from a group in India which is descended from Africans brought there by Indian and Arab merchants in Medieval times.
My personal theory is that it is very, very easy to get to America - it's difficult to leave! (using pre-modern technology).
Concerning reporting the existence of the New World to the Europeans, I wouldn't have considered it myself until the mid-1400s when Europe was less barbaric. The Chinese knew about it, visited it, and turned their backs on the opportunities. The Byzantines may well have made a few voyages of discovery. Third century Romans from Mauritania probably made it to near Cahokia, and so on.
Several weeks ago I was in a Hopi village when a little girl of about three (could have been Hopi or Zuni) came in the room. Her facial features were exquisitly Japanese.
I have been around Indians all my life but I have never before seen such purely oriental features, a tiny sculpted geisha. I had heard the Japanese/Zuni theories before but that clinched it for me. It was jawdropping. It was like looking back hundreds or thousands of years into history.