So, when you arrive to camp registration, you pay for the week/remaining available days up front and then when you return the credentials you will (eventually, as in a week or so) receive a refund for unused nights. They do have a three-night minimum, so staying for just one night would cost you a pretty pricey $84.
The regs also say, "To camp in one of the aircraft camping areas, you must be an active EAA member. If your aircraft is parked in this area you will be charged for camping." As we have always camped on field as long as the airplane was there, I cannot speak to whether this means you pay for as long as your plane is there (perhaps one of the HBC guys will chime in) but the credentials go on the tent, not the plane and there's nothing that says you can't pack up and move the plane to HBP if necessary, you'll just need to coordinate with a HB volunteer to taxi it over.
There are usually plenty of planes without tents in HBC (ours has fallen into that category when we've camped with friends in Vintage), but I cannot recall HBP having tents. It's quite a bit further from the amenities, even if it is allowed.
I can neither confirm nor deny how stringently the camping credentials are enforced on Sunday, or if you make a new friend that isn't an immediate family member but who offers to share their camping footprint how closely that will be scrutinized...