The more I think about it, the more it fits.
Anakin was a good man before his fall to the darkside.
Something happen to Anakin to made him turn to darkside.
After ROTJ was released, Lucas was asked about Luke and Leia's mother/Anakin's wife. Lucas said that he did not give her character much thought.
So, at the time of making the OOT, Luke and Leia's mother/Anakin's wife was more likely not the cause of Anakin's fall to the darkside like she was in Revenge of the Sith.
like I wrote before: Anakin was a good man before his fall to the darkside and Something happen to Anakin to made him turn to darkside.
I think that the fall into a nuclear reactor shaft in The second draft of The Empire Strikes Back screenplay was the Original cause of vader turning to darkside.
I think Darth Vader's OOT Origin is this:
After Anakin fell into a nuclear reactor shaft, Anakin found himself with mutations, radiation sickness and extensive cancer and Anakin turned to the darkside to save his own life. Years afterward, Obi-Wan saw what happened to Anakin and try to help Anakin, but it was too late. Obi-Wan and Anakin battled each other. The Battle ended with Anakin falling into a molten pit, inwhich the back of Anakin's body is burned which kills off the last remaining parts of Anakin Skywalker in his mind.
this Darth Vader's OOT Origin fits the OOT. In ESB, after battling vader, Luke jumps and falls into a shaft which may have been foreshadowing Anakin's fall into a nuclear reactor shaft. then, In ROTJ, luke is "on his back in pain" from the emperor's lightning may have been foreshadowing Anakin "on his back in pain" as his back was burned in the molten pit.
Also In ROTJ, the Dying vader throws the emperor into a reactor shaft which may have also been foreshadowing Anakin's fall into a nuclear reactor shaft or a type of Revenge for the falling into the nuclear reactor shaft which turned Anakin to darkside. The Original name for Return of the Jedi (the tittle is about Vader returning to good) was Revenge of the Jedi (this tittle fits the Revenge for the falling into a nuclear reactor shaft idea).