Reading Mosiah 27, to my surprise I saw a chiastic structure here as well. I have never seen it pointed out before, but it certainly includes a lot of the elements and criteria for a chiasmus that I know of.
A: 11 And as I said unto you, as they were going about rebelling against God, behold, the angel of the Lord appeared unto them; and he descended as it were in a cloud; and he spake as it were with a voice of thunder, which caused the earth to shake upon which they stood
B: 12 And so great was their astonishment, that
C: they fell to the earth,
D: and understood not the words which he spake unto them.
E: 13 Nevertheless he cried again, saying: Alma, arise and stand forth, for why persecutest thou the church of God? For the Lord hath said: This is my church, and I will establish it; and nothing shall overthrow it, save it is the transgression of my people.
F: 14 And again, the angel said: Behold, the Lord hath heard the prayers of his people, and also the prayers of his servant, Alma, who is thy father; for he has prayed with much faith concerning thee that thou mightest be brought to the knowledge of the truth;
G: therefore, for this purpose have I come to convince thee of the power and authority of God, that the prayers of his servants might be answered according to their faith.
G': 15 And now behold, can ye dispute the power of God? For behold, doth not my voice shake the earth? And can ye not also behold me before you? And I am sent from God.
F': 16 Now I say unto thee: Go, and remember the captivity of thy fathers in the land of Helam, and in the land of Nephi; and remember how great things he has done for them; for they were in bondage, and he has delivered them.
E': And now I say unto thee, Alma, go thy way, and seek to destroy the church no more, that their prayers may be answered, and this even if thou wilt of thyself be cast off.
D’: 17 And now it came to pass that these were the last words which the angel spake unto Alma, and he departed.
C': 18 And now Alma and those that were with him fell again to the earth,
B': for great was their astonishment; for with their own eyes they had beheld an angel of the Lord;
A': and his voice was as thunder, which shook the earth; and they knew that there was nothing save the power of God that could shake the earth and cause it to tremble as though it would part asunder.
Could it be that Mormon knew about the chiastic structure of Alma's words to Helaman on the plates of Nephi and decided to create a chiasmus himself when abridging the record of Alma's conversion in the Book of Mosiah?