1. Why do the Oankali want the humans to become self-sustaining?
They want it so that the humans can provide everything for themselves. After the nuclear war they don't have faith in humanity. The Oankali want to mix with them in order to secure the best outcome for both species.
2. Why does Lilith admire Curt and Gabriel?
She admires them for staying true to themselves.
3. Why did the Oankali choose to create the environment that is a replica of a tropical forest on an island?
They did it because a forest is a place where people can find food and shelter if they are resourceful.
4. Why does Lilith doubt that they are on a space ship and wonders whether they are on Earth?
Lilith ...