Here we have two instances where singular pronouns are first used to refer to whosoever, but then the text switches to plural pronouns. In his editing for the 1837 edition, Joseph Smith changed the plural pronouns to singulars: their to his in verse 34 and they to he in verse 35. But, it should be noted, Joseph left the plural these at the end of verse 34 in “and these shall enter into my rest”. (Admittedly, replacing these with he or this in this clause would have been awkward.)
As discussed under 1 Nephi 17:48, whoso and who(m)soever can take either the singular he or plural they. As noted in that discussion, there are other passages where the number switches. Thus the critical text will restore the original mixture in pronominal number here in Alma 12:34–35.
Summary: The original text allows whosoever to take both singular and plural pronouns, even within the same sentence; restore in Alma 12 the original use of their in verse 34 and they in verse 35.