The 1888 LDS edition accidentally replaced the conjunctive now with and. This edition was set from the 1879 edition. In that edition, the preceding two verses (29 and 30) and the following verse (32) each begin with And, which may have influenced the 1888 typesetter to accidentally set And instead of Now in verse 31. The 1888 edition never served as a copytext, so this particular and at the beginning of verse 31 was never copied into any subsequent LDS edition. Either reading would work here, so we follow the earliest reading, now.
Summary: Maintain the conjunctive now at the beginning of Alma 2:31 (“now when Alma had said these words …”).