Maybe I'm wrong, but I thought that the novelization for ANH said that Vader and Obi-Wan fought and Vader fell into a volcano. I don't remember the radiation stuff from TESB.

I would think that in ANH, the Emperor wasn't really going to be the main villain, or really even that important of a person. He is only mentioned in the novel, and there he is protrayed as a shadowy figurehead, and basically the military and officals like Tarkin actually held most of the power. Even Vader listened to Tarkin, and ultimately took orders from him, even though he was sent by the Emperor. Then, starting with TESB, the emperor becomes a more powerful and important figure.

Also, there had to of been more between Anakin/Uncle Owen that the prequels gave us. clearly Owen had a big grudge against Anakin, and something more than just one single meeting had to of happened.