Does topological sort exist for any complete directed acyclic graph?
Yes it exists and the existence of a linear ordering not only shows the existence of the topological oredering over the vertices but also that this ordering is unique. It is not hard to prove that the graph represents a linear ordering using the facts that the graph is complete and acyclic. That should be a good exercise for you (proving a relation is a linear ordering means to prove it is reflexive, asymmetric and transitive).