Time to lay this one to rest. A friend of mine works in the movies(such as Saving private Ryan, The fifth Element, Harry Potter etc). His Grandad was Bill Welch(the construction manager for the original Star Wars trilogy), and so he has a lot of film memoribelia(how u spell that?) including story boards and concept art, even a part of the floor of the Millenium Falcon. Also he has a original script for ROTJ. And so, the layin to rest. The lava pit dual conversation was simply cut from the script and not even shot. Apparently Lucas got rid of it pretty early on.
from the ROTJ Novel Obi Wan talking to Luke
"When I saw what had become of him, I tried tp dissuade him, to draw him back from the dark side. We fought... your father fell into a molten pit. When your father clawed his way out of that fiery pool, the change had been burned into him forever - He was Darth Vader without a trace of Anakin Skywalker. Irredeemably dark. Scarred. Kept alive only by machinery and his own black will"