𝓞 is not extant here, but there is clearly room for the subordinate conjunction when in the lacuna. When Oliver Cowdery copied from 𝓞 into 𝓟, he initially omitted the when (probably because he had started to write a new line in 𝓟). Virtually immediately Oliver supralinearly inserted the when (there is no change in the level of the ink flow for the correction). The when seems necessary here.
Summary: Accept in Alma 43:48 the corrected reading in 𝓟 with the inserted when (“and it came to pass that when the men of Moroni saw the fierceness and the anger of the Lamanites …”).