Oliver Cowdery initially wrote an ampersand in the printer’s manuscript; then soon after (with no change in the level of ink flow) he crossed out the ampersand and supralinearly inserted the pronoun he above the line. His correction was probably in agreement with the reading of the original manuscript (no longer extant here). Without the correction, he would have ended up with a sentence fragment. Although there is evidence in the original text for such fragments (see the discussion under Enos 1:3 regarding Enos 1:1–2), most instances of sentence-initial when- clauses are completed by a main clause.
Summary: Accept in Helaman 7:12 Oliver Cowdery’s correction in the printer’s manuscript as the probable reading of the original manuscript: “and now when Nephi arose / he beheld the multitudes of people”.