JOEL - The difficulty in understanding some of these scriptures is that the word "God" can be singular or plural. It can refer to one of the members in the Godhead, or it is sometimes used in place of the word "Godhead" and means all three members as one team called "God". When Amulek says there is no more than one God(Alma 11:28-29), he is talking about the Godhead.
Therefore when you see a scripture that says "this is the doctrine of Christ, and the only and true doctrine of the Father, and of the Son, and of the Holy Ghost, which is one God, without end" (2 Nephi 31:21); the word "God" in this context can be replaced with the word "Godhead".
Later in Alma, Amulek talks about the son of God(Jesus) who shall come and save them(Alma 11:32-35). When he says that the son of God is the "very eternal father of heaven and earth" (Alma 11:39) he is talking about Jesus Christ who, along with God the Father, can also carry the title "Father" of our heaven and earth. When Christ introduced himself to the brother of Jared He said, "I am the Father and the Son" (Ether 3:14). The prophet Abinadi said that the Messiah would be "the Father…and the Son" (Mosiah 15:3).
Christ can be called "Father" because of His role as creator of this universe. He is the "Father" of our salvation(Mosiah 5:5-8; 15:10-13; 27:25-26). And He can be called Father because of the authority God gave him to act for the Father(John 5:30, 43). This is evdent at a few times in the scriptures when Jesus is speaking, He talks as if He is the Father(See D&C 29:1 and 42).
When the scriptures talk about the Father, and the Son, and the Holy Ghost being "one"(3 Nephi 11:27), they are not talking about their physical nature. They mean that they are one in mind, one in purpose, and one in character. The D&C scriptures(D&C 121:32, 132:18-20, 37) that talk about "gods" are not talking about the God or Godhead of our universe, but the gods of other universes past, present, and future.
Understanding the exact nature of God is difficult when you try to make sense of all the scriptures in all the Standard works on this subject. I think the reason for this is that in this life our finite minds will never be able to comprehend all that God is and does, including the prophets who have lived or are living on this earth. We will all have to wait until we are again in His presence and actually experience His glory and power.
The main subject of the scriptures on baptism(Moroni 8:22-23)of course deals with infant baptisms, for which Mormon(writing to his son Moroni) declares that such "availeth nothing".
In verse 25 of chapter 8 Mormon states;
"And the first fruits of repentance is baptism; and baptism cometh by faith unto the fulfilling the commandments; and the fulfilling the commandments bringeth remission of sins;"
Children do not understand how to fulfil commandments and so are not accountable for any sins against those commandments. And as it says in verse 19, "Little children cannot repent...". If they can not repent they don't need baptism.
When Mormon says "and also all they that are without the law...baptism availeth nothing"(verse 22), he is not talking about people who never get a chance to hear the gospel in this life; he is talking about those people who never do become accountable or can not comprehend "the law" in this life because of mental retardation or some other type of mental handicap. The LDS Church handbook of instructions states;
“Persons who are not accountable and cannot knowingly repent need not be baptized, no matter what their age."
So it appears that those who pass through mortality without having become accountable retain the status of innocence spoken of by Mormon and therefore do not need baptism.
God will take care of those who are not accountable;
“And, again, I say unto you, that whoso having knowledge, have I not commanded to repent?
“And he that hath no understanding, it remaineth in me to do according as it is written.” (D&C 29:49-50.)