The flaw in the Diana-was-murdered conspiracies is that, if the foolish woman had put on her seat-belt, as her bodyguard did, she would have survived the crash, as he did. In fact, since she was in the rear seat, unlike the bodyguard in the much more vulnerable front seat, she very likely wouldn't even have been seriously injured.

It's a great pity that this aspect of the case wasn't emphasised much more in the sequel to the case - it might have saved the lives of a lot of people. But I suppose that pointing out that she only died through her own fault would have been seen as out of line with the hagiographical approach that so characterised the whole episode.