Dentist to the rescue! There is very little evidence to suggest that amalgam fillings are harmful in pregnancy, and common sense would dictate that the further along you are, the less the risk. There is even less data for white fillings (i.e. even less information as to whether they are safe or not). The biggest release of mercury is during removal of an old amalgam filling. If you did not have an old amalgam filling removed (i.e. it was a new hole or a white filling), the risk is even smaller.
If it is any sort of reassurance, I worked all through my pregnancy in a busy NHS practice, removing and placing dozens of fillings a week, handling raw mercury and breathing in the fumes. A much higher exposure than you having one filling. And DS was 100% OK.
I personally would prefer not to provide dental treatment to pregnant women, as although it is generally very safe, if there were to be a problem with the baby you would always wonder if it were due to the dental treatment. However, I would always provide appropriate emergency and preventative advice.
In short, I understand your concerns, but think you don't need to worry purely on a safety level.
OTOH I am very 
on your behalf that your dentist provided treatment you had not consented to. You had agreed to a temporary filling. He put in something else without your agreement. He might have told you what he was doing, but had not properly discussed risks etc, and you were unable to respond. This is what I would be complaining about -if I could be arsed-.