Does a valid Topological Sort have to be unique?
No.
If two nodes have no dependencies between them, their relative order can be swapped.
Example: Socks and Shirt. You can put Socks first or Shirt first.
If a topological sort is unique, what does it imply about the DAG?
It has a Hamiltonian Path.
There must be an edge between every consecutive pair of vertices in the sorted order (v1->v2->v3...).