Moroni only referrers to “Mahonri Moriancumer” as the brother of Jared and not by his full name in the Book of Ether. Other than part of his name which is in (Ether2:13), the place by the sea to which the people of Jared did pitched their tents, before they came to the Americas. Thus, it is implied but not stated that this is the name of the Brother of Jared. If we look at (Alma 8:7), we see that most places, lands, cities, and even small village were named after the leaders who first possessed them. Thus, we have the Brother of Jared name in verse 13 of Ether Chapter 2 but not his full name.
Note: Joseph Smith gives a baby blessing to the son of Elder Reynolds Cahoon and the baby was given the name “Mahonri Moriancumer”. What is not known to me is this, did Joseph Smith already translated this part of the Book of Mormon or not, if so, then the church all ready knew the Brother of Jared’s name because this would have all ready been published in the Book of Mormon in the Book of Ether, but not have realized it at this time in History.