The earliest text here reads whomsoever. The nominal clause headed by whomsoever serves as a direct object in the larger sentence and is conjoined with the preceding thee etc. Thus the text means ‘I will bless whomsoever shall be called thy seed’. In prescriptive grammar, the decision between whosoever and whomsoever is supposed to be according to its grammatical role within the nominal clause, not its role within the larger sentence. In accord with this rule, the 1920 LDS edition replaced whosoever with whomsoever earlier in the text:
In Mosiah 26:22 the original whosoever serves as the direct object within the nominal clause (just as whom would be the direct object in the corresponding relative clause “the person whom ye receive”). According to the prescriptive rule, the correct form in Alma 3:17 should be whosoever since whosoever is the subject in the nominal clause (as in “the person who shall be called thy seed”). The 1840 edition made the grammatical change to whosoever, but it was reversed to the original whomsoever in the 1908 RLDS edition. The 1920 LDS edition did not make the change to whosoever, although according to the strict prescriptive rule, it should have. Of course, the critical text will in each case restore the earliest reading, thus ignoring this rather esoteric prescriptive rule that not even the 1908 RLDS editors nor the 1920 LDS editors were able to fully control. Also see the nearby discussion under Alma 3:10, where the choice in editing was more transparent. For a complete discussion, see under pronouns and which in volume 3.
Summary: Maintain the original occurrence of whomsoever in Alma 3:17, despite its supposed ungrammaticality (according to a rather difficult and quite artificial prescriptive rule of grammar).