DMPS doesn't give yeast and is a chelator so it must be the DMSA itself. DMSA is known to lower neutrophils which are the white blood cell that combat yeast. That is a possibility, I honestly don't know.
ALA has been shown to help lead toxic patients but never been proven to chelate lead. Another option for you would be to finish mercury chelation with ALA and when your all done use EDTA. You can't use EDTA when you have mercury.