I am not quite sure I understand your. Regarding the millions (perhaps billions) who lived and died without hearing or understanding the Christian message over the entire history of the earth, the authentic Jehovahs’ Witness position is that they will be given another chance (“a second chance” in your words) at some point? Is this correct? If so, I think this theory can restory justice to your original claim.
Perhaps I can explain a little further.....there is no injustice I assure you.
In the 'big picture' we see human creation as God's crowning achievement...the last of his creative works.....mortal beings of flesh and blood who were given God's attributes and qualities. They were to become his representatives on this planet as caretakers and zookeepers....only the animals would have no cages.
Humans alone were granted free will as part of the package of being "made in God's image". Even though they were mortal, God provided the means for them to live eternally on earth on the provision that they keep one stated law. They were not to partake of a specific tree in the garden that God had made his own, under penalty of death. The fruit of every other tree was available to them, so that this tree held no special interest for them as food because they had more than they needed already. God's provisions were generous and bountiful.
Then a rebel spirit masquerading as a serpent entered into the picture and began to question the woman about "that" tree. He planted seeds of doubt in the woman's mind and so out of self interest, she broke God's command....and she did not die right away, so she figured that the snake must have been right. When Adam returned, she offered the fruit to him. He was persuaded to eat as well and as soon as the deed was done, they discovered a strange feeling that came over them....one they had not experienced before...shame at their nakedness. Sin had begun to take its toll already. God's response was to evict them from the garden and to bar access to the only means they had available to keep them alive....."the tree of life". Angels barred the way to that tree probably right up to the time of Noah, when God dramatically changed earth's landscape in a global flood.
What did their actions mean for their children? Kicked out of the paradise that God had prepared for them, life outside the garden was hard work. From eating of the abundance of fruit, they now had to work for their food. Made doubly hard now because God cursed the ground and eating bread meant planting and harvesting grain in soil that produced very little yield. This was entirely their own fault, but their children were born into this circumstance through no fault of their own.
We are prone to sin because of a genetic inheritance that we cannot fix. But right away in the garden, God stepped in with a solution to their dilemma.....Genesis 3:15 foretold a seed who would come and put everything right. A redeemer would pay the price and set them free from the curse of sin and death, but only after being dealt a 'heel' wound by God's adversary.
Since God knows that humans are prone to sin, he does not judge us too harshly. But is eager to give all humanity a chance to attain to the perfect physical condition with which they were originally created. Adam lost that for his children and Christ came to get it back by paying the debt.
No one who has lived or died in the history of mankind will be treated unfairly by the most loving God.
If Christ will resurrect "both the righteous and the unrighteous" after he has established his Kingdom on earth, then we can rest assured that adults and children alike will be given a second chance to get things right with God.
Those who lived and died in ignorance will not be condemned but will have a thousand years to prove themselves. Revelation 20:6 speaks of those who will rule with Christ in heaven for the full term of the Kingdom's rule, acting as rulers and priests for millions of earthly subjects.
Those alive on earth at the time of the last judgment however, will not be shown as much compassion. Why? Because we are living in a judgment period right now.....the "sheep and the goats" are being separated as we speak and once their designation is confirmed, there is no changing of their status. (Matthew 25:31-46)
Just as those on board the ark in Noah's day had no control over the time when God closed the door, so the door of opportunity will be closed when Christ appears as judge of all mankind. Just as Noah warned the people of his day about the approaching cataclysm, so God's servants are giving due warning to all, but according to Jesus, it will also fall on deaf ears. Just as the people ignored Noah's warning back then, they will ignore the warning again for all the same reasons. (Matthew 24:37-39)
In this model, will the infants who died be allowed to live another life and grow up sufficiently old to hear the message and then be able choose to accept or reject it in this second reincarnation?
It isn't a reincarnation, but a full bodily resurrection, just as Jesus raised his friend Lazarus from the grave....and yes there is no reason why God will not allow all children that opportunity along with their parents.
Will the retarded in this life, be mentally whole in this second life they are given, such that they are able to hear and understand and then accept or reject this hearing of the gospel in their second reincarnated lifetime?
Absolutely. God will not bring disabled people back to his new earth. All were healed of their sicknesses and disabilities by Jesus and his apostles, which was a foregleam of what was to come. Such disabilities are the result of genetic imperfection due to Adam's sin.....God knows the heart of all the living as well, not just what appears on the outside. He also knows what that person would have done without their disability. We can be sure that there will not be any injustice done to anyone.
Do I understand your mechanism of “second chance” correctly?
All the resurrections performed by Jesus were a full return of life to the deceased, and a reunion with their overjoyed family. This is what we anticipate when all loved ones are reunited in the new world to come. I have so many to welcome back.
I know that LDS' have a very different slant on things, but this is what we believe.