These are great questions, and while I would love to try to answer each of them, I don’t think I can, but here are some thoughts that I hope might help.
You’re right in saying that the common answer to why God sends people to hell is that he is just. You’re not alone in feeling that this answer is sometimes unsatisfying. Thinking about hell is unpleasant, especially for those of us who have lost loved ones who don’t seem to have been believers when they died. This is very difficult.
Why would a good God create a place where people are punished forever, let alone send people there? In terms of length of punishment, as you say, wouldn’t 50 or 100 years be enough? Wouldn’t it be enough to punish someone for as long as they sinned?
Let’s start by thinking about the mathematical equation: 80 years sinning should equal 80 years punishment. You’re right that in the Old Testament, God instituted a law that the punishment should be equal to the crime and no greater. This seems fair and just. I wonder though, whether a sinner who lives for 80 years really does *only* sin for 80 years. The only way a sinner can turn from sin and serve the living God is by the work of the Spirit in them to change them. Since this good work of the Spirit will not be available in hell, what makes you think the sinner, in hell, won’t continue to sin? The picture of the condemned sinner in hell isn’t that they are truly sorry for what they’ve done and want to serve God, since this is only possible by the work of the Spirit. I’m sure the sinner in hell is sorry they are there, but not sorry in the sense that they want to change because they’ve offended God. If they’re sinning in hell, they have to pay off *that* sin as well, which means they will never ‘pay it off’.
Despite all that, I’m not sure God works by equations, though they seem to make sense to us. The reality is that sin is very serious to God. In fact it’s so serious that he sent Jesus to take the penalty for our sin, that we might be forgiven. Jesus is God the Father’s only Son, and he sent him to die to save us from the penalty of sin (and from the power of sin). That shows how seriously God takes sin. And it shows us how much he loves us. Romans chapter 5 verse 8:"God shows his love for us in that while we were still sinners, Christ died for us." To us, sin doesn’t seem such a big deal sometimes. We wonder why God doesn’t just wipe it out. But God sees sin as it really is, a violent, destructive, evil force, which seeks to deny him his place as Lord over his creation. It must be dealt with, and dealt with permanently. Even if we don’t see it now, the reality of hell shows us how serious sin is.
Another thing to think about is the issue of fairness. I agree that sometimes it seems unfair that God would send people to hell, but I wonder if that’s coming at it from the wrong direction. The Bible tells us that everyone has sinned and turned away from God, refusing to worship him as God and instead worshipping false gods and doing evil things (see Romans chapter 3 verses 10-18). What we all deserve, you and me included, is to be punished. If anything’s unfair, it’s that God allows anyone to be completely forgiven, through Jesus death on the cross. That means we don’t get what we deserve; it means we get forgiveness and eternal life, rather than judgment for our sins. That’s kind of unfair, but in a good way! And the gift of forgiveness is available to *anyone* who will put their trust in Jesus and follow him.
I hope this goes some way to answering your questions, or at least keeping you thinking through these things carefully. Don’t forget that Jesus instructed his disciples to go out into the world, in Matt 28:18-20, teaching people about him so that they could be saved. God wants that message of forgiveness preached to the ends of the earth, that ‘unfair’ message that shows us both how serious sin is and how much he loves us.