God himself is outside of the laws of time. A thousand years is like a minute to him. All the time. Continuing long, or indefinitely. So the words everlasting and eternal are not taken from our perspective of the words, but from the nature of the words themselves.
Eternity has no beginning, and no end.
Remember, it doesn't say that they are one, it says they are one God, or one Eternal God. Unless Zeezrom was correct, and Amulek a false prophet, I don't see how the Church of Jesus Christ of Latter-day Saints can believe both. Alma, and Nephi must have been mistaken, and worshiped a false god. Just like modern Protestant Christians, they too cannot be exalted to the fullness of the glory of God. But if this is true, then the Book of Mormon has false teaching in it.