Is the Mahayana shunyata same as the Theravada papanca?
Yes, more or less.

Although, strictly speaking you are making a category error.

Shunyata refers to the fact that everything is an abstraction, simplification, subjective observation, and that in fact everything appears from interaction of a bunch of processes (known in this context as "causes and conditions") - including the process of perception. Shunyata and Sankhara are closely related. 

While Papanca refers to the process by which we take that empty stuff, those appearances, and because of ignorance assign to them the status of real, objective, clearly delineated things.

Papanca is something that can be stopped through Buddhist education. But Shunyata cannot be stopped, it's a fundamental law.

In one sense they are opposite: Papanca is "bad" (because when mind does Papanca - it's confused) - but Shunyata is "good" (because when mind sees Shunyata - it's enlightened).

You are right about Nirvana. It's empty when you think about it as the object of your mind. But when you actually realize that everything is Shunyata and stop clinging to things because of that (including clinging to the concepts of Nirvana and Shunyata) - now that's real Nirvana which is not empty.