Concerning your question above, it doesn't start with the ten commandments actually. The story begins at the beginning (Genesis.)
God created the earth to be "good." If you notice, the food He gave to Adam and Eve was not animal's flesh, but rather fruit. For this reason, I believe God originally created animal life (including humans) to eat vegetation only. I believe that is good in the fact that no animal (including humans) died during this short-lived time. I personally believe that the first animal killed was after Adam and Eve disobeyed God's one commandment to them.
and that some animals began to grow carnivorous characteristics.
So, when God created life on earth, it was not His intention for them to start killing each other.
When Cain killed Abel, God demanded an account from Cain, asking him what he had done. Cain replied with the famous words "Am I my brother's keeper?" To me, this shows that God did not
create people with the purpose of killing other people, but rather, humans through their free will, influenced the future of the earth in a very negative way
Humans had gotten so wicked after awhile that God decided to destroy most of His Creation, but allow a remnant to survive through Noah. Why? Because it hurt Him emotionally that humans were so evil.
That was not His plan for humans to be. After the ark landed, God allowed humans to eat other animals' flesh.
Now, did God know all this way before He created the earth? Yes. However, God thought it worthwhile to create life on earth anyways, because He loves His Creation.
Way before Moses' time, God chose Abram and promised him that he would bless all peoples through him. I think that's so cool! Even though my ancestry is not Jewish, I am not jealous, but rather it blows my mind how God, who allows people to do their own thing, chose a man who I believe sought and loves God (Abram) and promised to bless all people through his offspring!!!
Again before Moses' time, after Noah, there were the Noahtide laws, that are basically for everyone. They include not hurting people (by killing, stealing, committing adultery.)
During and after Moses' time, God stipulated that any foreigner who wants to seek God, should be allowed to do so. This means that it is basically up to every person to decide if they want to follow God or not.
So, to directly answer your first question:
"If G-d wanted everyone to obey him and worship him (and he does), why did he only give the Ten Commandments to one small group of people, in an isolated area in the world?"
He judges people based on all their "equation." Just because other people did not have the Ten Commandments, that does not mean that many did not seek and find Him where they were (which was not where Moses was.)
If you read Jesus' words accounted in the Gospels of the Bible, He doesn't mention hell for those who don't believe in him.
He does say that he is the way, the truth, and the life. However, when Jesus does mention hell, it is interesting that hell is emphasized as the place where those who don't help other people go. Please research hell in the Bible, and one can see that.