Here the earliest text reads whomsoever. Prescriptive grammar requires the subject form whosoever since whosoever serves as the subject in the nominal clause proper: that is, “whosoever suffered himself to be led away by the Lamanites” (just as in “he suffereth himself to become subject unto man in the flesh” in 2 Nephi 9:5). For a brief discussion of this prescriptive rule, see under Mosiah 26:22. Here in Alma 3:10, the nominal clause itself serves as the subject of the larger sentence, so there is no question that the prescriptively correct form is whosoever, not whomsoever. Even so, the critical text will restore the earliest reading, whomsoever, since it appears to be intended. For a more complicated case involving whomsoever, see the nearby discussion under Alma 3:17.
The 1840 edition made the grammatical change from whomsoever to whosoever in Alma 3:10, with the result that whosoever has continued in the RLDS textual tradition. The 1920 LDS edition introduced this change into the LDS text. The 1840 edition also changed the number of the verb, from the plural were to the singular was, a change that has followed the same textual history as the change to whosoever. The nominal clause has the singular himself, which agrees with the singular him in the following independent clause (“and there was a mark set upon him”). The plural were seems to be due to the immediately preceding plural noun Lamanites. For other examples of this type of subject-verb agreement based on proximity, see under 1 Nephi 13:23 (“a record of the Jews which contain the covenants”) and Jarom 1:14 (“the record of our wars are engraven”). Also see the general discussion under subject-verb agreement in volume 3. The critical text will restore the original were here in Alma 3:10.
Summary: Restore in Alma 3:10 the original relative pronoun whomsoever and the plural were,despite their ungrammaticality.