I camp in my van, is it possible to not camp on the island just fish and such then leave the island to sleep at night?

Yes, that is one of the unique things about it, that the free ferry allowed easy access to the island and the experience is different for everyone based on when you come, how long you stay and what you do. You can hike or bike, fish or boat. You can easily fish in the day, leave and come back in the morning a go again.

Nope. They have a very small parking lot on the mainland for guests to park and get faried overver to the island. There is no overnight parking available. They do have cabins and tent camp sites available on the island.