Here scribe 2 of 𝓟 initially wrote the plural transgressors, but then he immediately erased the plural s, giving the singular transgressor. The singular was undoubtedly the reading in the original manuscript. Either the singular or the plural is theoretically possible here, so there would have been little motivation for scribe 2 to have edited the plural transgressors to the singular transgressor.
Summary: Follow in Mosiah 26:19 the corrected reading in 𝓟: “because thou hast inquired of me concerning the transgressor”.