Here in Alma 6:2–3, we have two instances where an original whomsoever was changed to whosoever by Joseph Smith in his editing for the 1837 edition. In both of these instances, the nominal clause acts as the subject in the larger sentence while the who(m)soever acts as the subject within the nominal clause. From a grammatical point of view, the second of these conditions involving subject position requires whosoever. Even so, the original text of the Book of Mormon sometimes uses whomsoever in subject position; see, in particular, the discussion under Alma 3:10.
In this passage, Joseph Smith also worked on making the number for who(m)soever consistently plural. At first, he thought to treat the generic pronoun who(m)soever as a singular; thus he initially emended their in verse 2 to his. But then he reversed his emendation, undoubtedly when he noticed that in the following verse 3 there are many instances where plural pronouns are used to refer to the preceding who(m)soever: “their wickedness … themselves … those … their hearts … their names … their names”. Verse 3 also has many examples where the be verb is in the plural (namely, four instances of were), thus providing motivation for Joseph’s decision to emend two instances of was in verse 2 to were.
The critical text, of course, will restore the earliest reading for both instances of original whomsoever in verses 2 and 3 as well as the two instances of singular was in verse 2. Evidence elsewhere argues that the generic pronouns who(m)soever and whoever can be treated as either singular or plural (see the discussion under 1 Nephi 17:48). In addition, there is frequent disagreement in number between subject and predicate in the original text, as discussed under 1 Nephi 4:4 and, more generally, under subject-verb agreement in volume 3.
Summary: Restore the original nonstandard usage in Alma 6:2–3: that is, whomsoever rather than whosoever in verses 2 and 3, and was instead of were in verse 2; such usage is intended and found elsewhere in the original text.