I would assume that most criminals would throw the occupants out of the car (after taking their valuables), on the basis that the police would probably not have the resources to look into the crime. You would then have the car and money, and would most likely get away scott free.
Alternatively you would kill them both, so as no witnesses, and as you say probably rape the women first. However it does seem high risk to kill both or even one, when they are tourists, with the publicity that follows.
I thought the husband had just been lucky to survive initially. However changes in story, and unusual circumstances of hijack throw questions. It just seems preposterous that an intelligent man, would orchestrate such a crime.
Perhaps most likely explanation is that rather than fight to get Anni out, she was being held down, and he jumped out of moving car to save his own skin, and did not want to admit it??
As you say the taxi driver did not have much to gain from his accusations, as he has still ended up with an 18 year jail sentence.
Hopefully the truth will prevail for Anni and her family.