[And these things doth the Spirit manifest unto me]: “these things” are the future judgment of the world at the hands of the Old and New World apostles. Mormon is giving his testimony to the statement that he has made about the ultimate target of his writing.
[And for this cause I write unto you, that ye may know that ye must all stand before the judgment-seat of Christ…]: In keeping with his theme of judgment, he proclaims that his message to his future readers is that they too will stand before this judgment bar to be judged of the respective twelve who are given charge over them. We may not assume that the history of the Nephites is in the past and therefore of no import to us. We too must stand and be judged, and Mormon gives us his book so that we will be prepared for that great, awful, and awesome, day.