As discussed under Alma 12:8, scribe 2 of 𝓟 frequently omitted the conjunction and. Here in Alma 12:10, Oliver Cowdery supplied the and when he proofed 𝓟 against 𝓞. Clearly, either reading (with or without the and ) is possible in this context; thus there would have been no reason for Oliver to add the and on his own. The critical text will follow the corrected reading in 𝓟.
Summary: Accept Oliver Cowdery’s addition in 𝓟 of and at the beginning of Alma 12:10 (the probable reading of 𝓞, no longer extant here).