JOEL - From what I have learned,
infant baptism is not specifically taught as an acceptable or necessary practice in the Bible. Scriptures speaking of baptism associate it with faith in Jesus Christ, repentance, and forgiveness of sins, none of which are appropriate requirements for infants (Mark 1:4-5; 16:15-16; Acts 2:37-38; 19:4; 22:16; Rom. 6:1-6; 1 Cor. 6:9-11; Gal. 3:26-27; Col. 2:12-13; Heb. 6:1-6; 10:22; 1 Pet. 3:21). The first mentioning of infant baptism was not until around 240 AD by a Greek theologian named Origen who argued that baptism takes away the polution of birth.
Jesus Christ said of children, "Suffer little children, and forbid them not, to come unto me: for of such is the kingdom of heaven." (Matt. 19:14)
Requiring infant baptism is as though we are forbiding them to enter the kingdom of heaven unless they are baptized. What happens if they die before anyone has a chance to baptize them?
Perhaps the problem lies in the reasons why LDS baptize compared to the reasons why the Episcopal church baptizes people. Baptisms in the LDS church serves two main purposes; to figuratively wash away the sins of a repentant person(Acts 22:16) and to become a member of the Church of Jesus Christ. Since we believe that children are incapable of sin and therefore require no repentance, there are no sins for baptism to wash away, as is explained in the Moroni chapter you referred to. Our scriptures also tell us,
"And I also beheld that all children who die before they arrive at the years of accountability are saved in the Celestial Kingdom of heaven." (D&C 137:10)
Any child who dies before the age of eight is automatically saved in the highest level of heaven no matter who they are or what church they belong to. Therefore they don't need baptism for membership in the church because we know they are going to be saved in the Celestial kingdom of heaven anyway.
This is why Mormon in his letter to Moroni calls it a mockery before God; because both of the reasons for baptising a person do not apply to infants. It is as if we are telling God that we don't believe Him when He tells us that infants don't need baptism. Moroni also uses this example as a means of detecting the beginnings of apostasy within the church. And we need to understand the conditions under which he is speaking. He is very upset because the entire Nephite nation is beginning to dwindle in unbelief and are on a path to destruction. This is just one more gospel principle that they have corrupted along with many others.
So if he sounds a little harsh you need to consider his state of mind at the time he said it. We believe what he said, but we personally are not going to lash out at people and accuse them of mocking God. Those who believe in infant baptism will still be saved in some level of heaven, but they will "perish" in the sense that they will not receive exaltation in the highest kingdom of God in heaven.
The reasons you provide for infant baptism makes some sense, but according to LDS doctrine and what God has told us in our scriptures, they are not valid reasons.
Our infants do receive a blessing before the congregation shortly after they are born, which serves as one of the reason you gave for baptism; that the fellow members will be encouraged to help keep the child from evil and help him learn and obey the commandments.