The great faith of these men is evidenced in their willingness to separate. The smaller the group, the less likely to be able to withstand hostile raiding parties. However, it is also possible that the separation into smaller groups made the proselyting task easier, not only because they could speak to more people, but because they certainly could not be seen as a military threat in small numbers.
One of the facets of many ancient societies that may help us understand the dynamics of this visit to the Lamanites is the dichotomy between official tensions between different peoples, and the rules of hospitality. While the Lamanites and Nephites as a group certainly do not get along in the Book of Mormon, smaller numbers of people have no difficulty traveling from place to place and even entering the cities of their enemies. The official wars and armed conflicts appear to take place on a larger, more political level. On a personal level, people were people, and there appears to be nothing that we would term racial hatred that appears. There were certainly adverse beliefs that each held about the other, but in the end we have people who understand the importance of hospitality to travelers, particularly when they are obviously of no political or military threat.
Literary: "At the close of their harvest" refers to the harvest of souls, not the season of the agrarian harvest.