Normally you would not start DMSA until all amalgams were out, but other then the spots you are seeing, it seems you have them all removed.

I think it would be reasonable to contact the dentist who worked on those teeth and confirm that all of the amalgam was removed.

I have also heard of discoloring occurring from amalgam fillings, and perhaps that what you have.

As far as I know, it's OK to to DMSA if all amalgams are out, even if you have discoloration remaining.