Nitric will definitely dissolve some of the Pd.
The only thing I found to dissolve the copper without dissolving the Pd was a combination of sulfuric acid, water, and chromic acid. It's been 25 years, but I'm thinking it was 50/50 sulfuric (it may have been slightly weaker than this) with about 300g/l hexavalent chromic acid. This is a very, very dangerous solution, BTW. I don't think the solution was heated but it may have been. If so, that makes it even more dangerous.
After using the above solution, there will still be nickel of the back of the Pd point. This can be dissolved without dissolving Pd by using hot concentrated HCl.