I haven't managed to slog through the entire BoM; it's so deadly dull. Maybe Deep Shadow could tell us gentiles what the BoM says about people in America other than the purported ANE immigrants. Does it say anything at all about them?
The Jaredites are the one group that are clearly not ANE. There is a lot of evidence to suggest they were Asian in origin. Their empire was breaking up around the time that Lehi arrived in the Americas. Many LDS assume that the Jaredites were all destroyed in their climactic final battle, but this is another one of those generalizations that overlooks several statements to the contrary. Suffice it to say that the empire was destroyed, but that survivors scattered in all directions.
One group with ties to the Jaredites is the Mulekites. While they claim to be descended from a son of King Zedekiah in Jerusalem (a claim that may or may not be true), they have Jaredite names, customs, and even weights and measures.
About how many people does the BoM indicate were descended from BoM people in America?
Well, see, there's the problem: the BoM records that the Nephites were united with the Mulekites, and they were all called Nephites despite the fact that the Mulekites were the more numerous group (by how much, it does not say).
Even worse, the BoM states that at the time of the Nephite-Mulekite union, the Lamanites were at least twice as numerous as both the others combined. This is a curious statement, as the Lamanites seem to have had fewer women to start off with, and did not engage in polygamy to increase their reproductive rate, as the Nephites did. Thus, the best explanation for the Lamanite population boom is that they intermarried with preexisting natives. That the BoM records no such intermarriage explicitly should not be a surprise; it's a history of the Nephite half, not the Lamanite half.
Thus, it's likely that both sides intermarried with natives to increase their population. The populations of "Nephites" and "Lamanites" later are little more than ideological labels anyway; by the end of the book, it's more than likely that fewer than 10% of the people on either side are direct descendents of Lehi anyway.
Whew! I guess that covers the basics. Where can I fill in gaps for you, Auto?