Is it not time to let this one lie? Diana Spencer died a violent death in an RTA. That's all there is to it. She was a passenger in a speeding car driven by an individual who was at the time under the influence of prescription drugs and alcohol. When the vehicle careered out of control and crashed into the concrete pillars of a Paris underpass, three of its occupants were killed. One person survived, the only person in the car who was wearing a seatbelt. It is highly likely that, had Diana belted up, she also would have survived the accident.
And that's the end of it. Every year, across the world, thousands of people die in traffic accidents because they weren't wearing their seat belts. There are no hand-wringing, inquests, investigations or otherwise. It's just a shit deal.
Moral of the story: belt up! And let her rest in peace for goodness sake.