I've read through, the ideas here and, I do think it is interesting what people are coming up with here. I do like the idea of taking away Obiwan's lie. I wonder how your going to construct the Empire scene and, the jedi scene's with this in mind. Mabye something like this for the empire scene:
"Obiwan never told you what happened to your father"
"He told me enough"
"Did he? Then you know."
(luke doesn't say anything and, just moves farther down on the thing he's holding.)
"Obiwan killed your father"
"That's not true! That's not possible!"
"Join me and, we can (close-up of Luke) rule the galaxy!"
(luke jumps)

Though, that's just becuase, I read that you want him to say Obiwan killed his father... The "Then you know" part might be hard to find I don't know if Vader actually says that in the movies.... Mabye you could replace it with "search your feelings"...