"Someone made a progress report saying he has been chelating for 6 1/2 years now with minimal improvement."

That makes me believe that he has another more serious medical problem than mercury toxicity from amalgams. Some who chelate with ALA and DMSA also stay on very tiny doses. to get any meaningful outcome, one has to build up to a decently large dose. I am now chelating with 100mg DMSA and 200 mg ALA every 3 hours.

I have nothing against trying different things as long as they are safe. DMPS is known to have much more toxicity than DMSA.
I am not willing to try DMPS. It seems like a real gamble. Some people have been very badly damaged by DMPS.

http://www.dmpsbackfire.com/default.shtml

http://www.cfspages.com/chelation.html

http://www.jashbotanicals.com/articles/dmps_chelation.html