I think if Camilla only wanted to be Charles' mistress then she wouldn't have married him and would have made herself more discreet. Marrying the heir isn't the best thing to do so ... Perhaps at first she saw herself only as Charles' mistress but after his divorce she realised that he was the man of her life (I'm too romantic
). Or maybe she conclued that being his mistress is much more difficult than being his wife : you have to hide from everybody and you have to face the tabloids and the people in the morning
.