That's an awful lot of separate questions with a lot of different answers depending on which denomination of Christianity you ask. A comprehensive/detailed answer to all that would fill a book, or maybe more than one.
I'm still going to give it a shot because I'm bored, just know that this is definitely the bastardized version of all these answers.
Note I'm assuming with all these answers you mean, "according to Christians." Jews and Muslims would have very different answers for some of these.
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The question I ask, assuming the above two statements are true, is how are people who were born before Jesus' time considered to be saved? I mean, there was no Jesus then (well, I guess there was ALWAYS Jesus, but you know what I mean), so how would someone who was alive before Jesus going to be saved?
Prior to Jesus' coming, Jews made ritual animal sacrifice which cleansed them of their sin (thus making them effectively "sinless" which is the requirement to get into heaven). Once Jesus died for the sins of all mankind, those sacrifices were no longer necessary. So in this case, God's standard of morality didn't really change, just the method by which people atone for their sins.
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As I'm typing this, I just realized that I think I read before that when Jesus was sacrificed, he went down to Hell and allowed everyone there a chance at redemption
Jesus went to hell to endure the punishment for mankind, but he wasn't down there picking up converts.
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I'm guessing first of all, that hell must have been a pretty crowded place considering that maybe .0000001% of the population in the OT had a chance to meet God and be saved right?
Yeah, most people are ****ed. Some protestant denominations try to add the loophole that if someone never had the opportunity to hear about Jesus they're "off the hook" so to speak, but that's not the common view. By far the standard view is, "**** em, they're in hell."
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Does that mean that we can still get out of hell if we somehow ended up there?
The common view is, nope, once you're there, you're ****ed. No way out, just sit there and be sad forever. Occasionally some theologian will come along and argue that either people don't go to hell for not believing (they're just obliterated) or they have some chance after death to "opt in" to heaven instead of hell by choosing to believe after death, but those guys usually get chewed up and spit out by religious conservatives.
By far the common view is: If you've ever committed any sin, you go to hell. Also, literally everyone sins. You can get forgiven your sins and still go to heaven, by believing in Jesus and accepting his sacrifice for you. If you don't, you go to hell, and once you're there, you're stuck there forever.
Personally that strikes me as a convoluted, ineffective, and incredibly overly-vindictive system, but I tried to keep my editorializing out of the explanation as much as possible.