Political: Benjamin declares that he intends to name this people with a new name. Why does he do this? The very clear hint lies in his own description. He is naming them to " be distinguished above all the people which the Lord God hath brought out of the land of Jerusalem." That sounds fine, but what does it mean?
There are only three known groups who have come from the land of Jerusalem, the Lamanites, Nephites, and Zarahemlaites. It is quite unlikely that Benjamin is referring to the Lamanites in this statement. The Zarahemlaites have no kin ties to the Lamanites, and would know them as they knew all of their "others" - as outsiders and potential enemies (or perhaps trading partners). Even the Nephites would have no need of a name to distinguish them as "above" the Lamanites, as they have considered themselves superior from the beginning, with remnants of their low opinions showing in Enos 1:20 and Jarom 1:6.
Benjamin is giving a new name to the combined people of Zarahemla and the people of Mosiah, so that this new people will be greater than "all the people which the Lord God hath brought out of the land of Jerusalem" or precisely the separate people of Zarahemla and the people of Mosiah. Benjamin is making a bold political move designed to preserve the internal peace he as created, and he intends to perpetuate it by restructuring the political world inside the city of Zarahemla. While kin divisions will certainly remain, Benjamin intends to erase political divisions and unify the people.
This new naming is very clearly tied to religious principles. Benjamin specifically states that it can occur because "they have been a diligent people in keeping the commandments of the Lord." In the context of Words of Mormon 1:16-18, Benjamin sees this political move as specifically related to the resolution of the internal religious conflicts that Mormon summarized. We should not surprised by the combination of political and religious motives in the ancient world. The separation of the two is recent, for however much it defines North American perceptions. In the ancient world, however, reality was defined through religion, and the political reality was occupied because the leader could demonstrate (or at least claim) the sanction of God.
Archaeological: At this point it is worth remembering one of the most fascinating features of the Santa Rosa archaeological site that Sorenson proposes is a good candidate for Zarahemla. One temple had a very unusual feature. Under the plaster floor was found a distinct division between two types of gravel. In the words of one archaeologist:
"To the north the gravel was broken and to the south it was rounded. I supervised that excavation and, upon noting the difference, carefully searched the gravel, finding no mixture whatever. Not only does the difference suggest two sources of materials but it may be taken to imply two separate groups, each working on its section. Further, the medial line runs roughly east-west." (Brockington, Donald L. The Ceramic History of Santa Rosa, Chiapas, Mexico. BYU. New World Archaeological Foundation. 1967, p. 60-61.)
This suggests a very tempting scenario (albeit pure speculation) in the light of Benjamin's stated purpose for his discourse. This discourse will be given at the temple (v. 18). Suppose that this is not an old temple, but perhaps a new one, one that has been commissioned for this new enterprise. As part of Benjamin's coronation of Mosiah, we reidentifies his people, making of two a single people. As part of this ceremony, the new unity is physically symbolized by the ceremonial laying of the gravel, which is covered by a unifying plaster. The separation is no longer visible, symbolically buried in the temple, and conceptually buried in the new name.