Here in both 𝓞 and 𝓟, Oliver Cowdery initially wrote was instead of were. He was probably influenced by the immediately preceding singular cause. As discussed under Mosiah 10:14, there is extensive evidence that Oliver Cowdery tended to miswrite was in place of were, even sometimes making the error first in 𝓞, correcting it there, but then making the same error when he copied the text into 𝓟. Here in Alma 43:47, the ink flow for the correction of was to were was made with somewhat heavier ink flow. One possibility is that Oliver made this correction after reading the text back to Joseph Smith. On the other hand, the correction in 𝓟 is virtually immediate since there is no change in the level of ink flow. Thus one could also argue that the correction in 𝓞 was done when Oliver copied the text from 𝓞 into 𝓟, correcting 𝓟 first, then 𝓞.
Under Mosiah 10:14, I argued that evidence throughout the manuscript shows that Oliver Cowdery sometimes accidentally wrote was in place of were but that there was no independent evidence that he consciously emended was to were. In that discussion, I missed noting the difference in ink flow for the passage in Alma 46:33 (namely, the ink flow for the correction in 𝓞 is somewhat heavier). Consequently, there are at least two places where one could argue for conscious editing of was to were by Oliver, here in Alma 43:47 and in Alma 46:33. Nonetheless, there are numerous instances in the earliest text of the singular was being used with plural subjects, obvious instances of nonstandard subject-verb agreement that Oliver never corrected (see, for instance, the examples listed under 1 Nephi 4:4).
Summary: Accept in Alma 43:47 the plural were, the corrected reading in both manuscripts: “for this cause were the Nephites contending with the Lamanites”.