Here Oliver Cowdery initially wrote unbelievers in 𝓟 but then immediately corrected the word to unbelieving; he actually aborted the final s of unbelievers, erased it, then overwrote the final ers with ing. The Book of Mormon has six occurrences of the noun unbeliever(s) and four of the adjectival unbelieving, so either reading is theoretically possible here in Alma 16:17. There is no doubt that 𝓞 read unbelieving since there would have been no motivation for Oliver to have emended the text here.
Summary: Accept in Alma 16:17 Oliver Cowdery’s immediate correction, “that they might not be unbelieving”.