Oliver Cowdery initially wrote “even down to the time that Lehi left Jerusalem”, but then almost immediately he supralinearly inserted their father before Lehi. (The level of ink flow for the correction itself is unchanged, but the insert mark is written in heavier ink, perhaps after Oliver had redipped his quill.) Oliver was undoubtedly correcting 𝓟 to 𝓞 since normally the text refers to simply “the time (that) Lehi left Jerusalem” (eight times, including a nearby example in Alma 18:38: “from the time that Lehi left Jerusalem”). But there are two other occurrences of this expression where the name Lehi acts as an appositive to our father:
The critical text will maintain the fuller expression here in Alma 18:36.
Summary: Maintain in Alma 18:36 the corrected reading in 𝓟 with the added noun phrase their father: “even down to the time that their father Lehi left Jerusalem”.