As discussed under 1 Nephi 3:16, the plural “the commandments” is much more frequent in the Book of Mormon text than the singular “the commandment”, but the singular does occur (in 19 out of 146 cases). However, the original text has examples of only the plural “keep my commandments” (27 times), including earlier in this verse: “inasmuch as ye shall keep my commandments”. For the second occurrence in this verse, scribe 2 of 𝓟 initially wrote the singular commandment, but almost immediately afterwards he inserted the plural s (there is no change in the level of ink flow). There is one other example in the text where the scribe (in this case Oliver Cowdery) made the same initial error and virtually immediate correction while copying from 𝓞 into 𝓟:
For this part of the text, the 1830 edition as well as 𝓟 is a firsthand copy of 𝓞; thus 𝓞 undoubtedly read in the plural in 3 Nephi 18:14. For Alma 9:13 the critical text will accept scribe 2’s corrected plural reading in 𝓟: “inasmuch as ye will not keep my commandments”.
Summary: Accept the plural commandments in Alma 9:13; scribe 2’s correction in 𝓟 was very probably based on the reading in 𝓞.