"david prowse is actually saying "obiwan killed your father"
No he isn't. He said something further about how it is Luke’s destiny to join him. The Obi Wan thing is from a fake "what if" web site that was supposed to be as if there was an internet back in the pre-ESB days. It was a report from an “insider”. It was making fun of the prequel spoilers!
well, the point was that to keep the big twist secret no one knew what it was, up to the point where the actors where given phony lines to say during shooting.....
the only problem i have with what you're saying fardawg is that it would have to be something that would make luke react the way he does....it's a pretty extreme reaction for "c'mon, you know you want to".....