The bears on Niku are extremely shy. We've never even seen one.
In our experience, coconut crabs are not a threat to sleepers. The smaller land crabs and Strawberry Hermit crabs are much bolder. We found that they could be diverted by distracting them with garbage. Doesn't take much. A few empty Coke cans keeps them busy but the noise tends to keep you awake (clink, clink, clink).
Note: I get this information second hand from those of our group who have stayed overnight on the island. I haven't.