Does anybody know where the 2 channel version on the SACD came from? Is it a new remaster, an older remaster, is it an old copy of the original mix?

can't remember but it might be in this thread on SH.tv (http://www.stevehoffman.tv/forums/showthread.php?t=141294&highlight=DSOTM+SACD), or you may post the query there

It's a new remaster, but it's the same mix.
this link mainly about the MCH mix but it might interest you.

http://www.soundandvisionmag.com/interviews/445/tales-from-the-dark-side.html