Graphics Problem and NavMesh Obstacles

I am using NavMesh Agent components for path finding.

First question; when there is a crowded scene, characters start interlocking. For example, I have a scene with 500 characters with Navmesh Agents and they have same destination point.

At first:

At last:

What can I do to prevent this?

Second question (I think it is related to first question). When I use NavMesh Obstacles, some agents don't avoid from obstacles and pass through. All agents are clones of a prefab. What is the problem in this case?