Here in 1 Nephi 12:3, the 1911 LDS edition accidentally added a plural s to manner, probably because of its partial visual similarity to the following plural wars. Of course, “after the manners of wars” is clearly contrary to usage (in English as well as in the Book of Mormon), and thus this error was removed in the following LDS edition (1920). Of course, the text is consistent in its use of “manner of X” rather than “manners of X”, with 170 occurrences of the singular and none of the plural.
Summary: Maintain the singular number for manner throughout the text; there are no occurrences of manners in the Book of Mormon.