Do you have to stay 7 nights? That's what it says, but won't let me do 7 nights.

Got to book at least 2 nights ,and nothing over 14 days, after 14 days you can rebook a different site

I believe the 7 night mln is for the Villas, not sure about the cabins. I think all the SC state parks have this now. No min for the campground.

I've also booked tent sites for fewer than 7. That's quite a long time for a campground. Maybe there's an event?

Are you talking about rv sites. I have not booked cabins so I can't speak for them. I have however booked rv sites many times with less than 7 nights so I'm not sure why they are not allowing you to do so.