In the original manuscript, Oliver Cowdery seems to have initially written “if it was requisite with the justice of God”. (For the possibility that he may have written requisite as requisites, at least initially, see under Alma 41:2.) Sometime later, with heavier ink flow, Oliver corrected the indicative was to the subjunctive were. The was is not actually extant in 𝓞, but the final ere of the were, as a supralinear correction, is extant. As discussed under Mosiah 10:14, Oliver’s scribal practice argues that he did not intentionally emend was to were. He did, however, frequently write was instead of the textually correct were and then would later correct his error. Here in Alma 61:12, Oliver seems to have made the correction when he read the text back to Joseph Smith. The critical text will accept the corrected were of 𝓞 as the original reading in Alma 61:12.
In the construction “if it was/were”, the earliest text prefers were over was (29 to 2, excluding the case here in Alma 61:12). The two instances with was are not extant in 𝓞, but in both cases Oliver Cowdery was the scribe in 𝓟 and he did not replace the was with were:
In two other cases of “if it was/were”, an original were was changed to was but not permanently: in Mosiah 29:13, Hyrum Smith apparently thought to revise an original were to was, but Oliver, in proofing 𝓟 against 𝓞, restored the original were in 𝓟; the other case of change involves the 1906 and 1907 LDS editions and is discussed under Alma 17:22.
Summary: Maintain in Alma 61:12 the corrected reading in 𝓞, “if it were requisite”; Oliver Cowdery’s correction to were appears to have been made when he read back the text to Joseph Smith.