Thanks for your response.
Once I check the receipt and I see a per mile rate, what could I do at that point? Is there a way to loop around without starting the rental? IIRC you must exit once in the exit booth lane. I presume I would have to go check the car back in, pay for the shortest term rental and then go with another reservation.
Also, I have 2 return flights booked, one for my arrival airport (SEA) and one for GTF. So far, the price difference between the cheapest confirmed unlimited miles rate and my round trip rate is $1100, so I'm sort of hinging my itinerary on whether the Emerald Isle reservation would work.