Yes, there is approximately 50 percent mercury in dental amalgams that are called "silver fillings" by a lot of dentists. Your doctor may not be lying, but just misinformed like a lot of us. We have been hearing about "silver fillings" so much, that many people don't have a clue that one of the most toxic substances for the human body (i.e., mercury) is in this very common dental restoration material.
From what I read, it's rare that nickel is found in a typical mercury-silver dental amalgam. If nickel is there, it's probably a minor unintended impurity.