hard for me to believe that someone could survive a dunk in lava. instant melt
i mean really what makes most sense are the ideas they had going on during the development of ESB. that Vader suffered from intense radiation, a long fall, and/or substantial light saber dismemberment if i remember correctly.
in ROTJ when he's being electrocuted you can see his skeleton and if you catch at the right moment you can see that the vertebrae in his neck was rebuilt. where'd he get that?!