Be skeptical about any heavy metal test. They are not known for giving reliable information. The fact that you've had Amalgam fillings is a strong indicator that you almost certainly have a mercury problem (perhaps in addition to a problem with lead).
I've never heard of moss being used as a chelator. You can give what your doctor suggests a try, but I would do your own research about it first to see if your doctor knows what he's talking about. It's amazing how many doctors don't seem to have a clue about chelating.
As for DMSA and candida flares. It varies a lot from person to person. I didn't have huge problems, but I've heard from others who have. There's only one way to find out whether or not you would be affected badly by it...
There are quite a few people out there who describe improved health after replacing root canals. You could consider that too.