Even though DNA evidence suggests that Indians are not closely related to the Israelists and they have been here for 15,000 years or so? I'm not sure I want to hear a justification for that.
I can't believe this. You misread what I said, so I am forced to open up my argument.
I think that the Book of Mormon itself makes it clear that a significant proportion of population of the Americas is not from the line of Lehi, and therefore not descended from the Isrealites. For one thing there is the book of Ether in the BoM which discusses an earlier tribe of people. The other important thing to note that most Mormons and non-Mormons completely miss in the BoM is that "the Lamanites" is the name given to the dark skinned people that they find in the Americas. It is niether stated nor plausible that these people were actually children of Laman and Lemuel.
The other important thing to note is that all the light-skinned people were killed off. So there is little need to expect the Nephite bloodline to still be around.
I'm afraid that the 15,000 year old natives and the genetic code of the remaining natives are not sufficient to break the foundation of this religion. Sorry.