Initially in the original manuscript, Oliver Cowdery started to write “and also their liberty”. But after writing the ampersand for the and, he crossed it out, then wrote inline yea & followed by the rest of the text. His correction was therefore immediate. He seems to have been influenced by the lack of yea in the preceding text:
Since either reading, with or without the yea, is possible for the following that-clause, there would have been no motivation for Oliver to have consciously inserted the yea. The critical text will follow the corrected reading in 𝓞.
Summary: Maintain in Alma 43:9 the corrected reading in 𝓞, “yea and also their liberty”.