Setting NavMeshAgent.destination questions.

I'm doing some crowd simulations in Unity, characters lining up behind each other, but I seem to be fighting against NavMesh.

1. I'd like to have Left Guy line up behind the guy on the right. Currently I'm setting the NavMeshAgent.destination to a calculated point (the magenta vertical line) but NavMeshAgent.destination is adjusting itself to the red vertical line. How can I check the calculated point to check if it is a valid point on the NavMesh?

2. Imagine the wall was skinnier and the purple line was actually a valid point on the NavMesh. How could i detect and avoid having characters line up behind each other in this manner? (Like, the baked NavMesh shows a break, what calculations can I do to follow the edge of the NavMesh rather than skip over areas.)