While no amount of mercury is harmless , you only had one filling so your exposure was far less than most of us.
It sounds like the dentist killed the tooth . They always use the excuse that they had to drill too close to the nerve but I think the real reason is that they overheat the tooth by working too fast without taking breaks and not using cool water to counteract the heat caused by the drill.
Blood and urine tests are inaccurate because they don't indicate how much is stored and because fecal excretion is the main path of mercury removal.