Having President's Circle status makes no difference here
Keeping receipt for petrol purchase is a sensible thing to do since it provides fairly decent evidence that petrol was purchased for the vehicle, how much , where and when
With that, no issue. Alternatively wait until there is an agent to mark the car off before leaving
Not sure what you can do - If you have asked the location and it has said no to cancelling other than if mileage driven was short enough that it would not have been realistic to have used a whole tank, getting it reduced accordingly