Yep, its a bummer the situation you are in.
Sometimes the open box items work out, other times they don't.
Hindsight is twenty, twenty, but knowing there was impact damage on the camera, would tell me that this would not be covered under warranty.
For the salesperson to tell you that it was covered, well.......... uninformed employee.
If it were me in your shoes now, I would find the rep that sold it to you, and go from there.
By the way, how is the HC3 malfunctioning?