Urine tests do not measure body mercury. If it's a challenge test, it still won't measure mercury in the central nervous system, plus it can be dangerous in terms of redistribution effect, plus if it is done prior to Amalgam removal it will pull off mercury from the amalgams.
EDTA is a dangerous means of chelation.
Mercury exposure during Amalgam removal can be minimized through use of a dam and other procedures typical of a dentists who follows the procedures of Huggins or the IAOMT.
Exposure to mercury from removal can be taken care of through use of DMSA or DMPS according to the Cutler protocol over a course of several weeks.
And from there, you no longer have an ongoing source of toxicity in your mouth 24/7/365. You can begin to chelate. Again, I would recommend the Cutler protocol.