gfci's are current based, not potential based. The current measured on all legs must be accounted for, so for simplicity's sake lets say 100mA is going out on leg 1 the gfci expects to see approx 100mA on the neutral. If there is only 95mA on the neutral the gfci recognizes that 5mA of current has found an alternate path to ground and trips. (This is the ground fault, not so much there is a problem with the provided ground but current has found an alternate path to ground) This alternate path could be a person getting shocked, a march pump leaking current to the grounded chassis, a loose wire on the control panel making contact to ground, etc etc.
As far as updates go, I am compiling a large document and sets of drawings, The info has become a bit scattered in this thread. I'll start a new thread and put it all in there. Perhaps the wiki too, but that is new territory for me.