Oliver Cowdery initially wrote the text without the yea, but then later (with somewhat heavier ink flow) he supralinearly inserted the yea, perhaps when he proofed 𝓟 against 𝓞. The word yea is typically used to elaborate upon a subject that has already been referred to. In this example, the baptism of Alma’s converts has just been mentioned; the following clause describes where they were baptized and explains that after their baptism they were filled with the grace of God (presumably a reference to the Holy Spirit). There would not have been any particular motivation for Oliver to have independently supplied the yea here.
Summary: Retain in Mosiah 18:16 Oliver Cowdery’s inserted yea, the probable reading of 𝓞.