After re-reading my comments above, I must issue a caveat.

I'm just thinking out loud here. I'm relying on my astronomical intuition, not on a rigorous analysis. We are new to this system, and its located in a neighborhood we know little about. The system is full of surprises. I may be missing something, in fact I may be totally off-base. I'm just giving you my informed and educated opinion, but it is nonetheless just an opinion.

And then there's this (from the Wpkipedia article on Charon)

Simulation work published in 2005 by Robin Canup suggested that Charon could have been formed by a collision around 4.5 billion years ago, much like Earth and the Moon. In this model, a large Kuiper belt object struck Pluto at high velocity, destroying itself and blasting off much of Pluto's outer mantle, and Charon coalesced from the debris.[21] However, such an impact should result in an icier Charon and rockier Pluto than scientists have found. It is now thought that Pluto and Charon may have been two bodies that collided before going into orbit about each other.