These are all excellent questions arising from highly emotionally charged concerns. The problem is that they are somewhat misdirected. We should be considering the nature of the offense and punishment rather than their respective durations. It is a bit like saying "the death of one man can't pay for the sins of millions of men." But what if that one man was also God? Obviously, that changes things. You see, no good judge would get hung up on quantity at the expense of judging quality. Suppose a defense attorney said, "It is unfair for my client to be sentenced to 80 years in prison for a crime that took him less than a minute to commit." The severity of the punishment is primarily based upon the quality of the crime, not merely how long it lasted. The nature of the offense is what matters the most. What would we think of a judge who sentenced a man who stole 3 candy bars more harshly than a man who stole a car? We wouldn't be distracted by the fact that there were 3 candy bars and only 1 car. We would immediately think about the value of what was stolen. Why then should we assume that God would be more concerned with how long a person lived or how many sins they committed (quantity) rather than the nature of sin (quality)?
When we conclude that a human judge has acted unjustly, we are appealing to a higher standard of justice. The trouble is that when the judge is God, there is no higher court to which we can appeal. And how do we come to the conclusion that any verdict is unjust in the first place? We consider the offense and the punishment and determine that one is much worse than the other. Ideally, we want the two to be equal (unless the punishment applies to us; then we want it to be lopsided in our favor). On some level, we imagine what it would be like to suffer the crime, and then we compare that to what we imagine it would be like to suffer the punishment. In some cases, we don't have to imagine--we know exactly what it is like to suffer in these ways. Now, we can argue all day long about how we think God ought to respond to sin, but the reality is that none of us are in a position to imagine what it is like to be a perfectly holy being and suffer the crime of sin. And being completely incapable of empathizing with the offended party will inevitably skew our understanding of what punishment is just. If the defendant is female, it would not be smart to have a jury that is entirely male. The difference between God and man is infinitely greater than the difference between a man and a woman. According to Scripture: "There is none holy like the Lord," "Holy, holy, holy is the Lord of hosts; the whole earth is full of his glory," "Who is like you, O Lord, among the gods? Who is like you, majestic in holiness, awesome in glorious deeds, doing wonders?" (1 Sam 2:2, Isa 6:3, Ex 15:11). We know what it is like when people do wrong to us, and we are far from perfect. How then should God respond to sin? Only He is in a position to know.
Now, don't misunderstand me. I do not claim that anything I have said above makes the question of eternal punishment any easier to grapple with. It is absolutely terrifying. But I am convinced that it is not the sort of thing that is supposed to go down smooth with a sweet aftertaste.