In his editing of this passage for the 1837 edition, Joseph Smith changed the first participial clause to a finite clause by replacing the being there with was. His editing was motivated by the fact that the original subordinate clause here is never completed. As discussed under Enos 1:3, such participial fragments can be found in the original text. For two other examples, see the discussion under Mosiah 9:1–2 and Alma 4:11–12. The critical text will restore the instance of incompleteness here in Alma 10:31–32.
The 1892 RLDS edition accidentally omitted the subject pronoun he after the infinitive clause “to accuse Amulek and Alma”. The he is clearly helpful since without it one might misread the immediately following being as referring to Alma (“to accuse Amulek and Alma / being one of the most expert among them”).
Summary: Restore the incomplete participial clause in Alma 10:31: “now he being the foremost to accuse Amulek and Alma”; the original text has a number of such incomplete instances of sentenceinitial subordination; also maintain the he in “he being one of the most expert among them”.