What if someone is born in a polytheist family. He was told about islam but he chooses to believe in what his parents taught him.
Depends on many factors.
1- There is a duty towards God. Was he ready to think and reflect? If not, was that because he didn't want to listen, or because the Islam talker did it in a bad way? ( I think that all polytheist have thought whether what they are doing is right. But are they willing to consider it, that is the question.
2- When he was told about Islam, was it presented correctly? Was he convinced and said no I am going to live my life the way I want. I don't want to be that committed.
What if someone was muslim but became Hindu or Christian, will they goto hell for leaving islam ?
That is a hard one too. Again I am in no position to judge. But some things are apparent. Like if he is doing it just for fun than yes. Remember that this is based on that Islam is the right religion.
Leaving a religion for the other must be based on the right reasons. We are in the worst days now. There are many chaos in the world and you don't know who is true and who is not. Everyone is attacking everyone.
But for example if someone want to switch from religion to another.
First, what are the reasons you are leaving the religion. Are you sure about them? Did you listen to both points of views or just one? Does the religion you are going to have the answers to the problems? If yes, religion should be perfect, is it perfect.
Personally If I were to change my religion, there are some established facts in my head.
Like God is just, wise, and all knowing. God doesn't send us to this world without a purpose. Islam explains to me that purpose that I should worship God so that God helps me in following is commandments so that I go to heaven.
Does it makes sense? Yes. Look at people, all things and the hard works they do, does it go with them? NO. There is a purpose.
People doing the good deeds and who are going to heaven are very happy that they came into this life. They are happy to have problems in their lives because the more patient they are the more they are rewarded in heaven. So life for them isn't bad after all. One judging from the outside may say look at their miserable life. They enjoy nothing. I assure you that those people are happy 100000 times more than any other person in the world because they have acquired peace.
Personally, I don't think that all people will go to heaven, because God is just. One can't dismiss all his desires in order to go to heaven in the time where everyone is going there.
This is what makes no sense. Why does not believing in God and prophet justify eternal hell. I believe in god but I no longer believe in islam and Mohammad being prophet. Will I goto hell?
It is not just about believing in the prophet. When you say you are believing in the prophet, as if you are saying that you accept the Quraan as the word of God. And you accept all the prophets who came with the massage of Islam. And I do mean all the prophets including the prophet Jesus peace be upon him.
Islam is not a "religion" in the way we understand. It is a way of life. The way of life that God has sent down to mankind through the Quraan (in our age) and all the prophets. Islam provides to you with an explanation about all what you need to know in this life. It tells you what there is after life. It tells you how to live, how to treat others, how to love people, not being racist. It answers every question you have. It tells you how to reach heaven.
About your last question I can't answer you. No one can tell who goes to heaven who not. I am not in your mind I don't know your reasons. But one thing I can tell you that you will be accountable for all your actions and the reasons which drive it.
Anyways do you want us to discuss the reasons you no longer believe in Islam ?