1- By default, atheists deserve to go to Hell. However, if they did not reject God out of stubbornness or arrogance (but rather because of their non-religious upbringing, for example), then God may retest them and forgive them.
2- Before Islam, each nation had a prophet who showed them the right path. God sent 124,000 prophets in history. Those who followed the prophets were saved. Those who fought the prophets and defied God will be punished for that.
3- A believer has a much higher status in heaven. Even if a non-believer is forgiven, it doesn’t mean they go to heaven. They go to a place called A’raaf which is better than this world but lesser than heaven. And even if they make it to heaven by God’s mercy, they go to the lower levels of heaven whereas believers go to much higher levels.
4- Because those around the Prophet (s) receiving the verses, and those who would run society, were men. Hence the Qur’an sometimes addresses men because they were the immediate audience when the Prophrt delivered the verses (but it includes women of course).
As for who made God, since he is above space and time, he has no beginning, and when you have no beginning you are always there and don’t need anything to create you. We see this physical universe in motion and so we know it must have a cause. That cause cannot be mad up of matter because then it would need another cause—and you’d go into an infinite regression, which is impossible. There must be a starting point. That cause is above space and time and not made up of matter. He is God. He must exist by logical necessity, otherwise the universe could not exist. He is not made up of matter, he is above time, and so he has no beginning. When you have no beginning, you are always there and don’t need any other factor to make you