Without going into all the layers of questions at once, I'd like to look at one closely-related subject: What, if anything, did the authors of the New Testament say about this topic? Did they talk as though they thought the people of other nations and earlier times were condemned?
I'm only aware of a few passages of the New Testament that specifically discuss this type of question about people who lived and died in the years before Christ. The first is Paul's sermon in Athens, addressing idol-worshipers:
From one man, God made every nation of men, that they should inhabit the whole earth; and he determined the times set for them and the exact places where they should live. God did this so that men would seek him and perhaps reach out for him and find him, though he is not far from each one of us. "For in him we live and move and have our being." As some of your own poets have said, "We are his offspring."
Therefore, since we are God's offspring, we should not think that the divine being is like gold or silver or stone -- an image made by man's design and skill. In the past God overlooked such ignorance, but now he commands all people everywhere to repent. For he has set a day when he will judge the world with justice by the man he has appointed. And he has given proof of this to all men by raising him from the dead. (Acts 17:26-31, NIV, emphasis added on the parts that apply most directly).
Here Paul is speaking to idol-worshipers and talks about the whole sweep of human history up to his own day. Almost all of the people discussed are in groups that some people would assume are condemned; they lived before Christ's birth in lands that worshiped idols. But not once does Paul sound as if he thought they were automatically condemned. Paul could easily have said -- as some have said -- "God set some people to live in times and places before Christ's birth, because he knew he had not elected them to salvation, and there was no way they would reach out or find him." But Paul said nearly the opposite: that God did it so that people would seek him and perhaps reach out and find him, since he is not far from any of us. Paul might have said -- as some have said -- "They were guilty of idol-worship, a sin worthy of death, a sin against the law that is written on the human heart, so there is no injustice in condemning them even if they had never been given a law." But again, Paul said nothing of the sort; he said in the past God had overlooked such ignorance. We are accustomed to a system of laws where "ignorance is no excuse"; but according to Paul, God has a more merciful standard. Paul seems to assume that God overlooks sins caused by their ignorance.
Paul makes a point to say that God arranged history and nations so that people would seek him -- and Paul leaves open the possibility that they might find him. Paul said that no one was ever all that far from God.
There is one other passage in the New Testament that seems to talk about sin and salvation for people who lived in the times before Jesus' birth. Paul's letter to the Romans contains an extended section on the topic. This is a short excerpt:
Therefore, just as sin entered the world through one man, and death through sin, and in this way death came to all men, because all sinned -- for before the law was given, sin was in the world. But sin is not taken into account when there is no law. Nevertheless, death reigned from the time of Adam to the time of Moses, even over those who did not sin by breaking a command, as did Adam, who was a pattern of the one to come. (Romans 5:12-14, though the surrounding material is also worth reviewing for a more in-depth discussion)
In Romans, much of the letter has something to say on the topic; this is just an example. In the earlier chapters Paul shows how thoroughly the whole world was caught up with sin and subject to death. Here we see again a group of people that was ignorant of the law, and ignorant of sin -- and still died. Paul again touches on the subject of ignorance: that sin is not taken into account when there is no law. But that did not stop death.
Paul does not end with talk of sin and death. His real point is about Christ:
Consequently, just as the result of the one trespass was condemnation for all men, so also the result of one act of righteousness was justification that brings life for all men. For just as through the disobedience of the one man the many were made sinners, so also through the obedience of the one man the many will be made righteous. (Romans 5:18-19)
Paul spends some time discussing condemnation and salvation, discussing the people who lived before Christ; he never sounds as though he assumes they were lost. And -- strangely to our ears -- he never sounds as though he assumes Christ has nothing to do with them because of where and when they lived.
That leaves a lot of questions unanswered for the moment. Sometimes a question left unanswered may be better than a question answered incorrectly.
I'd be interested in other passages of the Bible that other people think about in connection with this topic.
I do have some more on this topic, but it comes from a completely different angle and will wait for another post.