Here the original manuscript is extant and reads “there was one Antionah … came forth”; the pasttense came agrees with the following conjoined past-tense said. The 1830 compositor replaced came with come, which was followed in the earliest editions. The 1849 LDS edition restored the original came (but without reference to either 𝓞 or 𝓟). The 1908 RLDS edition restored came by reference to 𝓟.
As discussed under Enos 1:23, the Book of Mormon text allows the past-tense verb form in the existential construction “there was (not) something ”, as in Helaman 13:2: “there was one Samuel a Lamanite came into the land of Zarahemla and began to preach unto the people”. The critical text will therefore maintain the earliest reading here in Alma 12:20 (“there was one Antionah … came forth and said unto him”).
Summary: Maintain the past-tense forms in Alma 12:20: “there was one Antionah … came forth and said unto him”.