In this passage we have two plural nouns, torments and flames, that have taken on singular forms at some time during the transmission of the text. In the first case, the printer’s manuscript originally read “when their torments shall be as a lake of fire and brimstone”. All the printed editions read in the plural, but Joseph Smith, in his editing for the 1837 edition, deleted the plural s here in torments. Ultimately, the 1837 edition maintained the plural torments. There is nothing grammatically wrong with the plural torments here, but Joseph’s emendation in 𝓟 may have been based on the consistent use of the singular noun torment elsewhere in the text (including six other instances involving the phrase “the lake of fire and brimstone”, each marked below with an asterisk):
Thus the occurrence of the plural torments in the earliest text for Alma 12:17 is unique. But since there is nothing really wrong with the plural usage, the critical text will retain it, especially since it is the original reading in 𝓟 as well as the reading of all the printed editions.
In the case of flames, the 1852 LDS edition replaced “flames ascendeth” with “flame ascendeth”, thus creating a singular subject flame for the historically third person singular verb ending -(e)th. But the original text allowed the -(e)th inflectional ending to take plural subjects. In fact, the original text prefers “flames ascendeth”, as explained under 2 Nephi 9:16. Thus the critical text will restore here in Alma 12:17 the original “whose flames ascendeth up forever and ever”.
Summary: Maintain in Alma 12:17 the original plural form torments; despite the uniqueness of the plural for this word in Alma 12:17, there is nothing inappropriate about it; also restore the plural flames, the original reading in 𝓟, especially since the expression “flames ascendeth” occurs elsewhere in the original text.