Here in Alma 12:21, the biblical scripture that Antionah is citing follows the language of the King James Bible:
Cherubims, the plural for cherub, is a double plural consisting of the Hebrew -im followed by the English -s (with 65 occurrences in the King James Bible but none of the purely Hebrew plural cherubim). And originally, the Book of Mormon text had the same double plural form, cherubims. Interestingly, scribe 2 of 𝓟 initially wrote the word as Cherabs (that is, as the purely English plural form for the noun cherub, spelled as Cherab by scribe 2); in other words, only the English -s was added to the singular noun. But then scribe 2 immediately corrected Cherabs to Cherabims, his spelling of the King James double plural cherubims. 𝓞 is not extant for this word in Alma 12:21, but 𝓞 most probably read as Cherubims, which scribe 2 misread as Cherabims (Oliver Cowdery, the scribe here in 𝓞, frequently wrote his u’s so that they looked like a’s). The 1830 compositor set Cherubims. In his editing of 𝓟 for the 1837 edition, Joseph Smith crossed out the plural s of Cherabims, but this change was never implemented in this passage in the 1837 edition. The change to cherubim was finally made in the 1920 LDS edition, but the RLDS text has retained the original King James double plural, cherubims.
The plural cherubims occurs two more times in the Book of Mormon text, and the editing of 𝓟 for the 1837 edition is similar except that in these two cases Joseph Smith’s change to the purely Hebrew plural cherubim was implemented in the 1837 edition:
The critical text will, of course, restore the original double plural cherubims, the consistent reading of the King James Bible and the earliest reading in all three cases in the Book of Mormon.
Summary: Restore the double plural cherubims in Alma 12:21 and in Alma 42:2–3; this plural for the Hebrew noun cherub is the form that the King James translators consistently used.