For the room only discount you would pay rack rate for the 15th and then the discount would apply to the rest of your stay, all under one reservation.
For the S/P/D, I believe your check in date needs to fall within the dates that the discount is available. Therefore, you could do a room only reservation, paying rack rate, for the 15th. Then, have a second reservation with the S/P/D package for the remainder of your trip.
That being said, we have only ever done RO discounts, so if I'm wrong, I'm sure someone will correct me.