HI Hannah, I don’t work at the park, but am fairly certain you can book multiple nights on a non reservable sight once you get it.  I used to camp a lot at Long Point and we would go up on the Thursday morning at 3am and line up to wait for the non reservable sights, then book them for the entire weekend. Unless they’ve changed the policy, I don’t think they have, you should be good.  Best to have a few back up plans though, just in case.  Sending you luck!