Was the American Civil War started inevitable or were there opportunities to avoid it?
No actions that we humans take are inevitable. All can be avoided. There were many ways the Civil War could have been avoided, but it is not surprising that they were not taken and the war happened.
The war could have been avoided most easily if one side or the other had simply given in. The North or the South could have given in on the issue of slavery in the territories. This would have prevented the war, but it is not likely that either side would ever have done this.
The war could have been avoided if Lincoln had simply allowed the South to secede. Instead, he chose to fight to preserve the Union. Again, though, it is at least somewhat unlikely that any President could have gotten away with simply allowing the South to secede.
In short, the war could conceivably have been avoided in a number of ways. However, it would have been difficult for the leaders to actually take the actions that would have avoided war.