Here Oliver Cowdery emended the plural justices to justice when he copied the text from 𝓞 into 𝓟. His emendation is undoubtedly correct. Not only did the scribes sometimes drop off the final s, but there are also a few cases where they accidentally added a plural s.
It is theoretically possible to consider this plural as meaning something like ‘the just decisions of God’ or even ‘justices [that is, judges] acting in God’s name’, but there is no usage in the Book of Mormon to support such interpretations; elsewhere we have only the “justice of God” (12 times, plus three more with an intervening adjective before God, as well as one example of “God’s justice”). A nearby example also refers to the division between the wicked and the righteous:
The parallelism between 1 Nephi 12:18 and 1 Nephi 15:30 also argues that the phrase “the justices of God” in the original manuscript is not an error for “the judgments of God”, especially since all Book of Mormon occurrences of “the judgments of God” involve judgments coming upon people rather than dividing them.
One possible source for adding the plural s to Justice may derive from the difficulty Joseph Smith and the scribes may have had in pronouncing the plural s when added to an unstressed syllable ending in the sound /s/. Two words that show signs of this difficulty are witness and wilderness. In certain cases, it appears that the original text may have read witnesses and wildernesses, but the /ßz/ ending was dropped because of the immediately preceding /ßs/ at the end of the base form. (See 2 Nephi 31:18 and Alma 34:26 for discussion of witnesses and wildernesses.) Here in 1 Nephi 15:30, the opposite may have occurred. Scribe 2 of 𝓞 heard the correct /¸ j^stßs/ but misinterpreted it as being /¸ j^stßsßz/, thus scribe 2’s plural spelling justices. In any event, usage elsewhere argues that this plural form is an error for justice.
Summary: Maintain in 1 Nephi 15:30 Oliver Cowdery’s emendation of justice for scribe 2’s justices; the plural reading seems impossible.