Here in the original manuscript, Oliver Cowdery initially wrote “neither was they slow”. Somewhat later, perhaps when he read back the text to Joseph Smith, Oliver emended the was to were. In 𝓞 the supralinear were was written with slightly heavier ink flow, but the crossout and the insert mark were written with heavier ink. Another possibility, of course, is that the change from was to were was a case of conscious editing on Oliver’s part. Note that the sentence begins with a were (“they were not lifted up in the pride of their eyes”), which agrees with the corrected reading for the following clause (“neither were they slow”). More generally, when we consider other cases in the text where Oliver changed was to were, we find that changes like these appear to be the result of his attempt to get the text down correctly rather than the result of a desire to grammatically emend the text. We generally find that Oliver sometimes mistakenly wrote were instead of was, which he corrected, but he did not emend genuine cases of was (even if ungrammatical). For a thorough discussion of this point, see under Mosiah 10:14; also see under subject-verb agreement in volume 3. The critical text will accept the corrected reading in 𝓞 for this passage: “neither were they slow”.
Summary: Maintain the plural were in Alma 62:49: “neither were they slow”; although Oliver Cowdery initially wrote “neither was they slow” in 𝓞, this reading appears to be a scribal error on his part that he later corrected after reading the text back to Joseph Smith.