In noun clauses headed by a wh-word such as who(m)soever, standard English grammar determines the case for the wh-word by its role within the noun clause, not by the role of the noun clause within the sentence. Here in Alma 43:10, whosoever is correct according to standard grammar because it serves as the subject in the noun clause (“whosoever should worship God”). But within the sentence, the noun clause serves as the direct object (inverting the word order, we have “the Lamanites would destroy whosoever should worship God”). The typesetter for the 1841 British edition replaced whosoever with whomsoever, perhaps because he noticed that the noun clause was the direct object. This change was rejected in the following LDS edition (1849). As noted in the discussion under Alma 3:17 and elsewhere, the critical text will follow the earliest textual sources in determining the case for wh- words rather than follow (when forced to) the sometimes arcane rules of prescriptive grammar for determining case. Here in Alma 43:10, the earliest reading happens to be in accord with those rules.
Summary: Maintain the subject form whosoever in Alma 43:10 since this is the reading of the earliest textual sources.