The 1830 compositor penciled in the plural s for threatenings in the printer’s manuscript and set the 1830 edition with the plural s. Clearly, the earliest reading “to breathe out threatening” seems unacceptable. The 1830 emendation is consistent with all other occurrences of the noun threatening in the Book of Mormon text; that is, elsewhere the text has instances of only the plural threatenings (nine of them), including four more with the phraseology “to breathe out threatenings” (never “to breathe out threatening”):
In the first example, we get the plural even with the preceding much; such usage is found fairly often in the original text (see under 1 Nephi 16:35 for “much afflictions”, Enos 1:21 for “much horses”, and the Words of Mormon 1:16 for “much contentions”). Also note that in 3 Nephi 5:5 there is no preceding determiner for threatening(s), just as in Mosiah 19:3. Further support for the plural threatenings in the phrase “to breathe out threatenings against someone” can be found in the King James Bible:
In Mosiah 19:3, the plural s for threatenings was probably accidentally lost during the early transmission of the text.
Summary: Accept in Mosiah 19:3 the 1830 compositor’s decision to add a plural s to threatening (thus “to breathe out threatenings against the king”).