I don't quite agree with your theory. I don't the stories were planned ahead of the rape/murder. That would mean it was all premeditated and I don't think that is the case. I think she was given an accidental overdose.....or raped/killed in the "heat of the moment" after refusing his advances.....
The stories were concocted after the fact, imho.
I think Sloot planned on having sex with Natalee, even if it meant rape. So, I think that part was premeditated. I doubt that he planned to murder her.