Here in the printer’s manuscript, Oliver Cowdery initially wrote them which, but then he corrected the them to these by overwriting the m with se, giving these which. The flow of ink for the overwriting is somewhat heavier. Oliver may have initially written them simply because he didn’t expect any following relative clause. But it is also possible, at least theoretically, that Oliver decided to edit the nonstandard them which to these which. However, in 29 other instances of original them which (where which is restrictive and directly modifies them), Oliver never once emended the them to these or those, as in five other examples where them is not preceded by a preposition (as here in Alma 14:15):
Thus it is very doubtful that Oliver’s correction of them to these in Alma 14:15 is the result of editing. Instead, his change is very probably a correction to the reading of the original manuscript.
In the 1840 edition, the these here in Alma 14:15 was changed to those. This change may reflect Joseph Smith’s editing for that edition, or it may simply be a typesetting error based on the much higher frequency in the Book of Mormon text of “those who(m)/which” than “these who(m)/which” (181 versus 6 in the original text). Another possible cause of the 1840 change is the occurrence in the preceding verse of “those who” (the reading of the 1837 edition, which served as the copytext for the 1840 edition); there the text also refers to being cast into the fire:
As far as Alma 14:15 is concerned, the second printing of the 1852 LDS edition introduced the those into the LDS text. This change is most probably the result of consulting the 1840 edition when the 1852 stereotyped plates were corrected for the second printing.
As explained under Mosiah 28:1, the critical text will in each case use the earliest textual sources for determining whether the text reads these or those. Here in Alma 14:15, the critical text will restore the original these.
Summary: Restore the original these as well as the original which in Alma 14:15: “to save these which had been cast into the fire”; the initial them that Oliver Cowdery wrote in 𝓟 is a simple scribal error that was soon corrected.