Paul Thomas (personal communication) has proposed that the singular is in “mercy claimeth all which is her own” be edited to the plural are (“mercy claimeth all which are her own”). One notes that the parallel preceding clause has a plural direct object (“all his demands”). By switching the is to are, we would provide a semantically plural interpretation to the direct object all in the second clause (“all which are her own”). David Calabro points out (personal communication) that a plural interpretation for all should be interpreted as meaning ‘all persons’ rather than ‘all things’. Note that the following clause specifically refers to those who will be saved: “and thus none but the truly penitent are saved” (Alma 42:24). Also note the language earlier in verse 23 (“and mercy claimeth the penitent’) with its reference to persons, “the penitent”. The grammatical emendation of is to are also implies that the which should be changed to who (“and also mercy claimeth all who are her own”) so that the reference to persons would be made clear in the standard text.
Yet the Book of Mormon text definitely allows the singular verb form in similar expressions; see, for instance, the discussion under 1 Nephi 17:30 regarding the phrase “and doing all things for them which was expedient”. Of course, here in Alma 42:24 there is no overt plural noun like people (or things), so the use of the singular is is technically not even ungrammatical. And even with the singular is, the all can still be interpreted as referring to people (or things). The King James Bible has examples of this usage involving things, such as “and all that in them is” (Exodus 20:11 and Acts 4:24); for discussion of this particular phrase in the Book of Mormon, see under 2 Nephi 2:14.
Summary: Maintain in Alma 42:24 the original, but somewhat awkward, expression “all which is her own”; here all is a plural that refers to persons.