Here in the printer’s manuscript, the word even was supralinearly inserted somewhat later (the flow of ink is considerably heavier and broader). Oliver Cowdery may be the scribe here, but since even was written quite awkwardly, one cannot be sure.
The phrase “yea even” is very frequent in the Book of Mormon text (with 190 of them in the original text), but of course yea without even is even more frequent. So there is no grammatical or stylistic motivation for inserting even here in Mosiah 23:13. This correction probably occurred when Oliver (presumably) proofed 𝓟 against 𝓞. For further discussion on the use of “yea even” in the text, see under 1 Nephi 10:3.
Summary: Accept in Mosiah 23:13 “yea even”, the corrected reading in 𝓟; this correction was defi- nitely done later, probably when 𝓟 was proofed against 𝓞.