In his editing of this passage for the 1837 edition, Joseph Smith emended the complex and incomplete parenthetical statement “now Tubaloth supposing that Coriantumr he being a mighty man could stand against the Nephites”. Joseph made two deletions here: he removed the clausal initiator, now Tubaloth (which repeats the preceding reference to Tubaloth, namely, “the king of the Lamanites whose name was Tubaloth”), and he removed the pronoun he after Coriantumr, thus ending up with the following simplified present participial clause: “supposing that Coriantumr— being a mighty man—could stand against the Nephites”. Cases of incompleteness resulting from parenthetical statements can be found elsewhere in the original text; for instance, the book of Enos (the first two verses) begins this way (see the discussion under Enos 1:3).
There are still examples in the text of the participial structure “X he being” (where X is a personal name), although other examples in the original text have been removed (again by Joseph Smith in his editing for the 1837 edition); see the discussion and examples under Alma 2:16.
Initially in 𝓞, Oliver Cowdery omitted the therefore that begins the completive statement in verse 17. 𝓞 is sufficiently extant that we can determine that Oliver originally wrote he, crossed it out, and supralinearly inserted therefore. Because he wrote the he once more inline, we can assume that this change was immediate (also note that there is no difference in the level of ink flow for the correction). The therefore is important here since it helps the reader overcome the incompleteness of verse 16.
Summary: In Helaman 1:16, the critical text will restore the incomplete parenthetical statement “now Tubaloth supposing that Coriantumr—he being a mighty man—could stand against the Nephites” (that is, the phrase now Tubaloth and the subject pronoun he will be restored to the text); the therefore that begins verse 17 will be maintained (the corrected reading in 𝓞).