OK first I want to say that I am terribly sorry to be reading this update but not at all surprised.
Sadly, the vet has confirmed our fears that there is far more going on that meets the eye.
Since you were not told that the chin had a significant injury surely that is classed as selling a chin under false pretences? This chin was not fit to be sold, has been in considerable pain/discomfort (as evidenced by the persistent chewing) & the injury should have been properly treated when the initial injury occurred - xrays should have been taken & appropriate treatment carried out (in this case, probably amputation).
Proudmom you should NOT be having to go through this at all or be paying for any of this - you should also receive a full refund for the price of the chin as well as all the vets bills being covered.