Here in the printer’s manuscript, Oliver Cowdery initially wrote a singular form for every noun that refers to kings, including in the clause “ye could always have a just man to be your king”, which agrees with two occurrences of a king elsewhere in this passage (“ye should have a king ... for you to have a king”). In this intermediate clause, Oliver seems to have first added the plural s to kings (there the s is added inline and without any change in the ink flow). Later he turned to correcting a just man to just men; in this case, the ink is lighter and the ink flow is smoother. It is possible that this second correction was done later, perhaps when proofing 𝓟 against 𝓞; the correction of king to kings, on the other hand, suggests immediacy. Clearly, the singular works perfectly well through the entire passage, so we should probably assume that the corrected reading is the reading of the original manuscript, no longer extant here. In fact, the plural usage in “ye could always have just men to be your kings” also works well enough despite the two nearby occurrences of the singular a king.
Summary: Maintain in Mosiah 23:8 the corrected plural reading in 𝓟: “if it were possible that ye could always have just men to be your kings”.