If you've seen two dentists that have recommending putting mercury fillings back in your mouth, maybe it's time to see a holistic dentist. Sounds like you're seeing the wrong kind of dentist!
The other thing that occurred to me, he seemed mighty quick to tell you it's "microscopic air pockets" (which is basically like saying Oops I did it wrong) and get his experienced partner to refill them. Then they tell you maybe they did it wrong again, and they want to put mercury back in. Somethings not quite right there, or maybe I'm paranoid.