First of all, I would like to restate that I do not claim to be a universalist. However, I have done some reading on it so I will take a stab at answering some of your questions.
Before I get started, my viewpoint is that we would all be best served if we as Christians were not so insistent on teaching as doctrine things unless the Bible is very clear on them. From my own personal experience I know of the great torment I went through as a little child after hearing the doctrine of eternal conscious torment preached. There were many nights I lay in bed as a small child pleading with God to not let me die, to please forgive me of my sins, to not let me be tormented forever. Very, very scary.
1. It has been claimed that the Church came up with the doctrine of eternal punishment, yet we find much of what Jesus said would have left that impression on His hearers, such as the parables of "the rich man and Lazarus", "the unmerciful servant", and "the sheep and the goats", &c. Jesus plainly warned of when and how eternal punishment would begin but never gave a hint of when or how it might end. How was the Church guilty of misleading people any more than Jesus if universalism is true?
You say that Jesus did not hint at an end to punishment. To me this seems very different than explicitly saying that there is no end to torment. You ask the question (stating as a premise) "if universalism is true", how is the church guilty of misleading people more than Jesus.
Well, if universalism is true, and the church says that it isn't, but that torment in hell never ends, this is "more misleading" because Jesus didn't say it would never end.
But I think that is a technical answer, rather than getting to the spirit of your question. I think you are saying that Jesus *implied* that the punishment was never ending, or at least that that is how the people listening would have heard it.
I am not sure that is true. But since I was not there, I cannot say for sure. I know that my own brain has been programmed from childhood to read "hell", with all that goes with that word, when I here certain passages that speak of wailing and gnashing of teeth, or being cast into outer darkness, or being cast into the lake of fire, etc, etc....
But what would the people of the first century have thought when they heard these words? Can you say for sure? When I try to deprogram my brain and read the old testament without hearing "hell" when I read Sheol, or the grave, etc, I come away with an impression that hell was not taught in these books.
So when Jesus used words like Gehenna, which WAS spoken of in the OT to speak of a place where dead bodies were going to be burned when Jerusalem was to be judged, maybe the people thought He was referring to the valley outside of Jerusalem (and maybe they were right).
Much has been said in other threads about Luke 16 and Matt. 25. I will let you review those if you want to see ways of looking at the Rich Man and Lazarus and the Sheep/Goats parables. You brought up another one. The Unmerciful Servant...
What would a first century hearer have thought of Jesus telling this story?
Perhaps they would have thought that when the kingdom came, that God through the Christ would judge people based on how they treated others. Doesn’t much of the Bible indicate that God will show mercy to the merciful, forgiveness to those who forgive, that with the same measure we use to apportion grace, God will use when He apportions to us?
My understanding of what the first century people were expecting was an actual physical kingdom to come when the Messiah arrived. That He would reign on David’s physical throne and would deliver Israel from her oppressors. Perhaps the idea of eternal conscious torment in a place called hell would not have occurred to them.
They had examples of God’s judgment prophesied in the OT, where language was used such as smoke ascending up forever, burning of corpses in the valley of Hinnom, unquenchable fire, etc. They had seen the fulfillment of these prophecies. Maybe they were more used to this type of language than we are. Is it possible?
Anyway, what has been my goal and continues to this day, is to try to learn to read the Bible without all the programming that has come with my upbringing, to read the Bible again “for the first time.” It’s not easy.
2. Not considering for the moment those who have never heard of Jesus, how will those who rejected Him in this life come to love Him while enduring the horrible fate of the lost, without being forced to do so by their circumstances? Forced love is no love at all. How are we to believe they would repent absent the compulsion of their suffering?
This is a very interesting question. I have considered this a lot. Since the Bible does not answer this question, I can only speculate. What I imagine is that people turning to Christ after they die will do so in ways similar to how I did it in this life. I rejected Christ for many years. I believe one of the main reasons I rejected Christ was that I did not like Him. I used examples of the pain He allowed innocent people in this life, and the idea of eternally punishing people for rejecting Him (or even for never having heard of Him). "How could one love such a God?", I thought. So I lived my life the way I wanted to. As one would expect, my life crashed and burned. Living my life without God, according to my own rules didn’t bring me the happiness I wanted/expected. It brought me pain, anguish, bitterness, frustration, etc. It came to the point where I started looking for answers that could only be found in God. I decided to give His way a chance ( as a last resort

).
I would imagine people, finding that they had rejected the truth throughout their lives, seeing the waste, how they had hurt people, would have to deal with the same kind pain and anguish I did. I did not turn to Christ because I was forced, I turned to Him because I thought He was the right answer. I turned to Him because I thought His way might be better than my way.
3. If the purpose of hell is for correction, what if Hitler at judgment day is cast into "outer darkness" (hell if you prefer) and promptly confesses Jesus as Lord, says he is sorry for all he did and promises to do better; will he immediately go to heaven? Upon your theory of correction, why or why not? Is he not corrected? And if a good moral man who helped people, was kind, but nevertheless a staunch atheist does not confess Jesus will he stay in hell while Hitler is in heaven?
Fortunately God will be the judge, not me. He will know if Hitler is truly corrected or not. But I think there is more to the story than just “confessing” Jesus as Lord. The Bible warns that a simple confession of “Lord, Lord”, is not enough in this life. Why would it be enough in the next? Perhaps Hitler will actually have to demonstrate faith. I expect even those of us who have surrendered to Christ in this life, but have not dealt with all of our sin issues, will find some unpleasantness when we have to deal with them at the judgment. We will all be judged according to our works, and our works will be tested with fire (1 Cor. 3). I am not sure exactly what process we must all go through to purify us for dwelling with the Lord, to rid ourselves of self. But to the extent we fail to do so in this life, perhaps there is a refining process at the judgment.
I find it hard to imagine a good, moral “atheist” in the judgment. With God standing in front of him do you think this good person would still reject the truth? Hard for me to imagine.
4. If a benevolent God can cast a man into hell for 1000 years, why not 10,000 or 100,000, or ad infinitum? When, in your mind, is the limit, or is there any?
If the purpose of sending someone to hell is for correction, the duration would be until the person is corrected (not measured in years, but in percentage corrected

).
If the purpose of sending someone to hell is for retribution, I cannot imagine a mortal human committing so much sin that it would take several lifetimes to make right, let alone eternity. But that is just my sense of justice. I must admit to being very biased here. I trust God knows what is right. I just believe that the Bible teaches that the wages of sin is death. Those who murder in this life, face a human judge with the potential of the death penalty. That is true whether you murder 1 person or 10,000. The maximum penalty we can bestow is death. Perhaps it is the same way with God...
You spoke in a previous post of the tenor of the Bible. To me, the whole Bible over and over again states this truth: You sin, you die. Why would God have misled people for centuries by telling them that they would die if they sinned, offered them life if they repented, and then at the judgment say ‘Sorry, when I said death, I meant eternal life in a lake of fire without the possibility of parole.”
5. If God cannot allow the wicked to suffer endlessly, how can he allow those who love Him to suffer so terribly in this life?
It is not a question of what God can allow, is it?
Are you suggesting that the fact that innocent suffer in this life is proof that the wicked must suffer endlessly? I suspect I am not reading your question correctly, sorry.
I believe God is going to make all things right. Do you think the person who suffered in this life will feel cheated unless the persons who caused their suffering are tormented eternally?
When we who are followers of Christ complete the process of becoming like Him, will we not also want God to forgive those who caused us to suffer, much as Christ wanted of those who crucified Him? Anyway, that is how I look at it.
6. What will be preached to those in hell who have heard and rejected the gospel? A different gospel? The Holy Spirit seems to have never saved a soul apart from the gospel. "The gospel is the power unto salvation". Who will preach what and when?
I do not know the answer to the question “who will preach, and when?” but I cannot imagine the gospel being different.
I grow tired of writing, and communicating is not one of my skills, but I wanted to take a shot at some of your questions. I know that Steve Gregg is teaching Friday night in McMinnville, Oregon on the topic of the 3 views of hell. Perhaps the teaching can be recorded and posted on his web-site. I am sure he will cover many of the types of questions you want answered. (And he does such a better job at expressing himself!)
Blessings to you, Homer!
Mike