If you really have to get it done and it's a choice out of Empress or Cerec, I would go with Cerec. I asked a biologic dentist and he said Cerec is the better one. But either is very expensive.
Yes, you should have every scrap of Amalgam removed. Any bit of Amalgam remaining could block proper healing. I heard of one person who only had only ONE tiny pit filling and that was enough to make them ill and keep them from getting well. I heard that only after every scrap of Amalgam is gone, can the body begin detoxing it. It can be a block to recovering properly. It is less exposure of course, but it not really conductive to proper post healing from thorough amalgam replacement.