The problem is how the people interact with each other as distinct, separate and isolated groups.
First, we have Mosiah leading a group of people out of the Land of Nephi. Keep in mind this is happening around 250 B.C., so it's been almost 340 years since Lehite landfall. They wander through the wilderness and come upon another group of people (who aren't "Lamanites"),
Now, if there were other indigenous people living in the land and the Book of Mormon peoples were a "subset" of these people, why would it be unusual or unexpected for them to find another group of people? When I'm driving through central California on I-99, I might not know every town I'm going to encounter, but I'm hardly surprised when I find a new one. Why weren't Mosiah's people expecting to find other people?
Then, upon encountering the people of Zarahemla, there is some form of communication that tells Mosiah that they had come from the Old World centuries before even though they don't speak the same language. So we have one group whose ancestors had come from the Old World 300+ years earlier, and they encounter another group of people (among all the other natives living in that land), and they don't speak the same language, but somehow this new group of people is able to communicate that their ancestors also came from the Old World 300+ years earlier!
Even if we believe that there were some people living in the the New World in 300 BC that knew about Jerusalem and the Old World, do we really believe that they could communicate this knowledge to someone who didn't speak their language in such a way that both would be able to figure out they were talking about the same place on the other side of the planet?
Then the people of Zarahemla discover that Mosiah had brought the brass plates with him. Did the people of Zarahemla have any written records? Based on Zarahemla knowing his genealogy by memory, apparently not. So it's been 350+ years since both groups came to the New World, the Zarahemlites don't speak the language and they aren't pre-Christian, but they get really excited over the brass plates (which are written in a language they don't understand).
Then Mosiah teaches them his language. Enough of them to the point of them wanting Mosiah to be their ruler. How does that happen? Do they set up tutoring centers all over the city to learn to speak Mosiaish, and the people sign up for a few hours a week, and they're all really motivated and gifted linguists that it only takes a few days, weeks or months until one day they all speak the same language (and even though there were more Zarahemlites than Mosiahites, it didn't occur to the Zarahemlites that maybe Mosiah's people should learn their language since it was their city).
Here's how Amaleki tells the story in the Book of Omni:
13 And it came to pass that he did according as the Lord had commanded him. And they departed out of the land into the wilderness, as many as would hearken unto the voice of the Lord; and they were led by many preachings and prophesyings. And they were admonished continually by the word of God; and they were led by the power of his arm, through the wilderness until they came down into the land which is called the land of Zarahemla.
14 And they discovered a people, who were called the people of Zarahemla. Now, there was great rejoicing among the people of Zarahemla; and also Zarahemla did rejoice exceedingly, because the Lord had sent the people of Mosiah with the plates of brass which contained the record of the Jews.
15 Behold, it came to pass that Mosiah discovered that the people of Zarahemla came out from Jerusalem at the time that Zedekiah, king of Judah, was carried away captive into Babylon.
16 And they journeyed in the wilderness, and were brought by the hand of the Lord across the great waters, into the land where Mosiah discovered them; and they had dwelt there from that time forth.
17 And at the time that Mosiah discovered them, they had become exceedingly numerous. Nevertheless, they had had many wars and serious contentions, and had fallen by the sword from time to time; and their language had become corrupted; and they had brought no records with them; and they denied the being of their Creator; and Mosiah, nor the people of Mosiah, could understand them.
18 But it came to pass that Mosiah caused that they should be taught in his language. And it came to pass that after they were taught in the language of Mosiah, Zarahemla gave a genealogy of his fathers, according to his memory; and they are written, but not in these plates.
19 And it came to pass that the people of Zarahemla, and of Mosiah, did unite together; and Mosiah was appointed to be their king.
Edited by cinepro, 30 April 2012 - 12:45 PM.