The context in which Camilla posed the question about hunting at the famous lunch was not whether or not Diana was going to hunt on a specific day. Camilla's query was a general one. She wanted to know whether Diana, after the wedding when she became Princess of Wales, was going to be regularly accompanying Charles when he hunted. Since this was an activity that she, Camilla, regularly shared with her lover, her reasons for wanting to know if Diana was going to be along seem pretty clear.
Even before the death of Diana this incident was reported in contemporary news sources including US Weekly and People Magazine. After the tragedy of the Paris car crash, I specifically remember reading it in none other than the Los Angeles Times.
If every story that originated with Diana needs to be considered spurious I don't understand why every denial that obviously originated with Camilla or her camp is automatically accepted as gospel.
For me at least, the story of the vetting lunch has the ring of truth to it. Diana did not hunt regularly before the wedding and she did NOT HUNT REGULARLY afterward either...just as she is reported to have assured Camilla she wouldn't.
For what reason would Camilla, a woman who had spent years in an adulterous relationship with the Prince of Wales, want to call his fiancée up for lunch in order to get to know her better? Diana is not the only young woman who would have become suspicious and even paranoid later.