Here in the printer’s manuscript, the 1830 typesetter, John Gilbert, crossed out the redundant words “therefore the brother of Jared”. Nonetheless, the preceding parenthetical statement (“now the number of the vessels which had been prepared was eight”) is so disruptive of the textual flow that the repetition of the full subject is very helpful to the reader (despite its stylistic unacceptability). Without the repetition, the text that follows the parenthetical statement seems completely disconnected, and the reader is forced to go back to the beginning of the verse in order to recover the subject. Even for the current text, it might therefore be better to have the repeated subject in Ether 3:1. The critical text will, of course, restore the original reading with “therefore the brother of Jared”.
Another case where therefore and a repeated subject (although as a pronoun) were deleted can be found in the editing for the 1920 LDS edition:
In this case, Oliver Cowdery initially omitted the “therefore they” in 𝓞 but later supplied it (probably when he read back the text to Joseph Smith). For a case where wherefore they was deleted (this time by Joseph Smith in his editing for the 1837 edition), see under the Words of Mormon 1:17–18.
Summary: Restore in Ether 3:1 the therefore and the repeated subject (“therefore the brother of Jared”); in this case, the intervening parenthetical clause is so disruptive to the text that this restoration, despite its stylistic unacceptability, might also be made to the current text.