VITAL QUESTIONS

For some time now,these questions have been on my mind and i will be so delighted to hear good answers from any one.

1: Does the devil really exist or is it the other side of God? Why has God not destroyed this devil with his overwhelming power or is he waiting for the devil to repent too?.

2: Before the coming of the Christian missionaries cum Christianity to Nigeria,our fore fathers were worshipping idols. If we most be Christians or born again before going to heaven does that mean our forefathers are all in hellfire?

3:Adam and Eve gave birth to Cain and Abel, as at that time there were only 4  people on earth i:e (Adam, Eve, Cain and Abel)
Bible said Cain killed Abel and was cursed by God. Now it was left with 3 people on earth i:e (Adam, Eve, Cain) 
Now according to the Bible,Cain traveled to a town called Nod,married and got Children with her.
Now my question is,where did the human Cain married emerged from?

 My Name is Zedicus Piro And You Know Say Words No Be Proble..