Here in the printer’s manuscript, Oliver Cowdery initially wrote “among all the prisoners of the Nephites”, a reading that will work. Oliver immediately corrected the text here by crossing out the Nephites and then writing inline the correct Moroni. The original manuscript is not extant here, but spacing between extant fragments allows room for only the shorter Moroni. Mormon himself was dissatisfied with what he wrote here since he added a clarifying or-phrase to the text: “among all the prisoners of Moroni or the prisoners which Moroni had taken”. The occurrence of Moroni in the or-phrase supports the preceding occurrence of Moroni in “among all the prisoners of Moroni”.
The corrective or is a very prominent characteristic of the original text of the Book of Mormon; for discussion and examples, see under Alma 22:22–23. The correction in 𝓟 suggests that in theory there could have been a corrective or for the phrase “among all the prisoners of Moroni”, namely, “among all the prisoners of the Nephites or of Moroni”. Yet when Oliver Cowdery made his correction here in 𝓟, he did not resort to such a device to deal with his own scribal error of the Nephites. Elsewhere there is an actual instance in the text where Mormon did correct the Nephites to Moroni by means of such an or: “and the armies of the Nephites or of Moroni returned” (Alma 44:23).
Summary: Maintain in Alma 54:3 the phrase “among all the prisoners of Moroni”, the immediately corrected reading in 𝓟 and the presumed reading in 𝓞.