I don't understand why Princess Diana had to die so that Charles could marry his mistress. His divorce from Diana left him free to marry Camilla in theory. However, there is a clause that prevents the monarch from marrying a divorcee (as Camilla surely was). That said, Charles was not actually the monarch at the time of his marriage so his union with Camilla, although built on shaky ground, is sound.
There is no coincidence either that I can see. Diana got into a car driven by a man who had drunk enough to make him three times over the British legal limit. She was not wearing a seat belt and she was driven at speed through a busy city street (admittedly, chased by reporters). The car crashed and she died of her injuries. Much the same has happened to thousands of other passengers in the same circumstances.
If, by an enormous stretch of the imagination, the Royals did consider getting rid of Diana, given how much she was adored, worshipped and fawned over, they would surely have thought twice. There would have been much more danger of the Royals being dragged down by such a revelation, than by a rather sad Princess maybe, and maybe not, marrying a Muslim.
I agree that she should be left to rest in peace and that her children should be spared more heartache. These allegations are a figment in the imagination of a vengeful and heartbroken father who cannot come to terms with his son's death and who had a huge axe to grind with the British establishment long before his son's death.