Here Oliver Cowdery initially wrote and at the beginning of the sentence; then virtually immediately he corrected the and to thus by supralinear insertion (there is no change in the level of ink flow). In the book of Mosiah, there are 15 occurrences of finite clauses beginning with thus (I include Mosiah 22:8 in this count), 29 with and thus, and hundreds with and alone. Although it is possible that the original text in Mosiah 22:8 could have read and thus, quite a few sentences in Mosiah begin with thus; consequently, there is no reason to reject Oliver’s virtually immediate correction here in Mosiah 22:8.
Summary: Retain in Mosiah 22:8 Oliver Cowdery’s probable correction to the reading of the original manuscript (that is, thus alone—not and alone or the combined and thus).