Re MCP. My understanding is that Cutler doesn't think it works for Hg.
Re. ALA + Cd. ALA (more precisely DHLA) is not known to be really good at chelating Cd; however its thiol/SH groups might be able to kick it up (mobilize it). That would put more Cd into circulation where it would find Se to bind with. This is just my theory.
Re Se. I know of a member of the yahoo FDC group who worked closely with her doctor re her dose of Se. She used high doses of it while chelating and got much improved rather quickly--not 5 years. However most of us will have to use more conservative doses of Se like you are using. That is roughly what I use.