Not all Christians agree on the consequence of original sin, so I am going to let you know right now that your answers here will vary. One of the great differences between Catholicism and other Christian faiths is the recognition that even with forgiveness, there is still penalty. If Adam truly repented his crime, then he was forgiven. From a Catholic perspective, I can tell you that if a person asks for forgiveness and they truly repent against a sin, they are forgiven by a loving God. A vengeful God, on the other hand, would not forgive.
However, there are consequences for sin— even forgiven sin. If you murder someone, for example, you may be forgiven, but you still have to pay the price for what you did. Jesus took on the penalty. He died for our sins. By His life, death and resurrection we are given the opportunity to go to heaven. God sent His only Son, out of love, to endure a torturous death, so we could be reconciled to Him. That is not the act of a vengeful God. True love, known to ancient Christians as agape love, is sacrificial in nature. That is the kind of love that God loves with. He loved us so much, that He actually condescended to join Himself to humanity so that He could save us from eternal hell (separation from God). Does that sound vengeful?
As to why the penalty for Adam’s crime was so severe, I do not know exactly, because I do not know the mind of God. It seems to me that God is using the disobedience of Adam to teach the whole human race obedience. Be that as it may, when Adam sinned, the grace He had been freely given from God was taken away for all his generations. That seems like a stiff penalty according to our puny standards, but we are not God. We do not think like Him, and our standards of justice and mercy are not the same as His.
As to your question about humanity up until Christ came-- those who died, who were just and loved God, were not banished to the punishment of hell. They went to a place of peace, known as Abraham’s bosem. This was not heaven because they had not received the sanctifying grace needed to dwell with God (nothing unclean can enter heaven). This place could not be considered punishment— not when you consider what true punishment is… hell. By the way, just because there were several thousand years between Adam’s death and Christ’s coming, does not mean that, to Adam, it seemed like thousands of years. There is no time in the spiritual realm.
