Safe heating food in a chafing silver plated dish that is showing copper underneath?
If you are considering heating any food in it, I would say no. 

Silver itself is not especially pleasant (wikipedia and a more detailed CDC study). So, if you have a dish that is losing its silver plating, it would be wise to be overly cautious than casual about using it for food again.

Is the base copper or brass? There is copper in brass anyway.

Copper has far worse toxicity potentially (wikipedia ) when exposed to acids. Cheese could contain some lactic acid though not in significant amounts.