Here Hyrum Smith initially wrote “if it were possible”; then he corrected the were to was by overwriting the ere of were with as. Later, when proofing 𝓟 against 𝓞, Oliver Cowdery corrected the was back to were: he crossed out Hyrum’s was and supralinearly inserted were. It appears that Hyrum himself decided to edit the subjunctive were to the indicative was. As discussed under Mosiah 10:14, the manuscript evidence is substantial that Oliver did not edit nonstandard was to were. In all probability, his correction to were here in Mosiah 29:13 was the result of proofing. This conclusion is supported by nine occurrences of “if it were possible” elsewhere in the text, but none of “if it was possible”. It appears that Hyrum initially copied the text correctly here in Mosiah 29:13, but then he decided that it were sounded ungrammatical and changed the were to was. This was the first instance where Hyrum was required to copy a subjunctive were. The only other subjunctive were he copied was the inverted were it found a few verses later in Mosiah 29:19: “and were it not for the interposition of their all-wise Creator”. In this second case of subjunctive were, Hyrum did not emend the were to was (perhaps because “and was it not” would have sounded like the beginning of a yes-no question rather than a conditional were-clause). The critical text will maintain the subjunctive were in Mosiah 29:13 (and in Mosiah 29:19).
Summary: Retain in Mosiah 29:13 the subjunctive were (“if it were possible”); Hyrum Smith initially wrote were in 𝓟; his decision to edit the were to was was later reversed by Oliver Cowdery when proofing 𝓟 against 𝓞.