Can we say cause/causes behind the world is/are eternal?
Nibbana is an ultimate reality, but the 'world' is a concept. What you call the world is merely seeing, hearing, smelling, tasting, feeling, thinking. These are just experiences. There is no arising of experiences in Nibbana. 


  If we say world has a beginning


But we cannot say this as it has no basis in reality. So any conclusions or questions you come up with based on this assumption will only take you away from the reality. It is not in the spirit of Buddhism to entertain such questions. 

Be mindful of the questions arising in the mind that only serve to please your imagination. They will lead you astray from the Buddhist path.