Here the present participle calling refers to what the Lamanites called the land, not what Aaron called it. The phrase “their fathers’ nativity” refers, of course, to the fathers of the Lamanites (presumably Laman, Lemuel, and the sons of Ishmael), not Aaron’s fathers. Oliver Cowdery wrote the possessive form fathers without any apostrophe, as was his normal practice. The 1830 typesetter correctly added the apostrophe to the end of the word. The 1852 LDS edition replaced the plural possessive fathers’ with the singular possessive father’s, probably unintentionally since in the second printing there was an attempt to correct father’s to fathers’ in the stereotyped plates. Unfortunately, a comma was added rather than the apostrophe, giving fathers, (an error which was perpetuated in all the later impressions that derive from those plates, from 1854 through 1877). When the 1879 LDS edition was produced, fathers, (that is, fathers with its comma) was misinterpreted as father’s, thus reintroducing once more the original 1852 error into the LDS text. This error was finally removed from the LDS text in the 1920 edition. The critical text will maintain the plural possessive form fathers’.
Summary: Retain in Alma 21:1 the plural possessive fathers’ since the text is referring to the fathers of the Lamanites.