"the Royal family had the motive, opportunity, and means to arrange a fatal accident. "
Let's look at that, shall we?
Motive: The Royal Family has a very long history of shameful situations, dodgy dealings and dubious relationships. Prince Philip's family had quite close connections with the Nazi party - for example, his youngest sister was at Herman Goering's wedding. But yet Philip is still alive. As I noted above, Charles' behaviour over Camilla was arguably much more damaging to the Royal Family than Diana shagging some random rich chap, Muslim or not.
Opportunity: Al Fayed and/or the Ritz owned a number of cars, so if you were going to tamper with the brakes you'd have to know well in advance which one they were going to take. The previous guff about the "computer chip" being removed and replaced and so turning the thing into a remote control toy is just that - guff and demonstrates a worrying lack of knowledge of how cars work. And if you didn't tamper with the car, how are you going to ensure it crashed?
Means: This is where it really does fall down. The secret service isn't staffed by robots. They're people. If you received an order to go and kill someone like Diana - someone who represents zero risk to the UK or its interests - would you follow it, or would you tell the person giving you the order to fuck right off? Moreover, the secret service doesn't act as some sort of enforcement squad for the crown - they have rules they have to abide by, they're not the sodding mafia.
But no, all that must be so much more convincing than "Drunk driver crashed his car while driving too fast." Because that never happens, does it?