The supralinear insertion of his in the printer’s manuscript appears to have been made somewhat later (the level of ink flow is uneven); Oliver Cowdery’s correction here may have occurred when he proofed 𝓟 against 𝓞. The his may have been initially lost here because Oliver had just come to the end of a line in 𝓟; having written the by at the end of the line, he may have been momentarily distracted as he started a new line and forgot that he had not yet written the his. Another factor is that Oliver had just written his in the immediately preceding text (“the truth of his words), which may have led him to omit the second his.
Here in Mosiah 17:20, either reading is possible in English (“by death” or “by his death”). Thus there appears to be no reason for Oliver Cowdery to have edited the text from “by death” to “by his death”. The critical text will accept the corrected reading in 𝓟 as the reading of the original text.
Summary: Retain in Mosiah 17:20 the corrected reading in 𝓟 (“by his death”), the probable reading of the original manuscript (no longer extant here).