𝓞 is extant here and reads “that they never would use weapons again”. When Oliver Cowdery copied from 𝓞 into 𝓟, he omitted the would, producing the awkward “that they never use weapons again”. Despite this difficulty, the 1830 typesetter copied the text as such, without the would. In his editing for the 1837 edition, Joseph Smith supplied this modal verb since it seemed necessary. Although it is theoretically possible that Joseph consulted 𝓞 here, the evidence elsewhere shows that he consulted 𝓟 but not 𝓞 in his editing for the 1837 edition. The critical text will, of course, maintain the reading of the original manuscript with its would.
Summary: Maintain the occurrence of the modal verb would in Alma 24:18, the reading of the original manuscript (“that they never would use weapons again”).