Here in the printer’s manuscript, Oliver Cowdery initially wrote “and now it came to pass”; then almost immediately he crossed out the now (the level of ink flow for the crossout appears to be unchanged). Either reading is possible, so there would have been no motivation for Oliver to delete the now except that the original manuscript didn’t have it. The intrusive now was probably prompted by the and now that begins the preceding verse:
Summary: Maintain in Mosiah 19:5 the corrected reading in 𝓟, “and it came to pass that he fought with the king” (that is, without any now after the initial and ).