The Alma quote is misleading as well. Here is what the BoM says elsewhere (must have been written when Joseph SMith was still trinitarian);
*THE BOOK OF MOSIAH
CHAPTER 15
How Christ is both the Father and the Son—He shall make intercession and bear the transgressions of his people—They and all the holy prophets are his seed—He bringeth to pass the resurrection—Little children have eternal life. About 148 B.C.
1 And now Abinadi said unto them: I would that ye should understand that God himself shall come down among the children of men, and shall redeem his people.
2 And because he dwelleth in flesh he shall be called the Son of God, and having subjected the flesh to the will of the Father, being the Father and the Son—
3 The Father, abecause he was conceived by the power of God; and the Son, because of the flesh; thus becoming the Father and Son—
4 And they are one God, yea, the very Eternal Father of heaven and of earth.
5 And thus the flesh becoming subject to the Spirit, or the Son to the Father, being one God, suffereth temptation, and yieldeth not to the temptation, but suffereth himself to be mocked, and scourged, and cast out, and disowned by his people.
6 And after all this, after working many mighty miracles among the children of men, he shall be led, yea, even as Isaiah said, as a sheep before the shearer is dumb, so he opened not his mouth.
7 Yea, even so he shall be led, crucified, and slain, the flesh becoming subject even unto death, the will of the Son being swallowed up in the will of the Father.
8 And thus God breaketh the bands of death, having gained the victory over death; giving the Son power to make intercession for the children of men—
9 Having ascended into heaven, having the bowels of mercy; being filled with compassion towards the children of men; standing betwixt them and justice; having broken the bands of death, taken upon ahimself their iniquity and their transgressions, having redeemed them, and satisfied the demands of justice. *