Maybe KurukÃ‚Â´s journey to the spirit world is a one-way trip.
This would be pretty much the same as dying. When an avatar dies, he goes to the spirit world and remains there forever. ThatÃ‚Â´s basically what happened to Kuruk.
Why is it so hard for him to find Koh?
Well, the spirit world is not like earth. I think about it as a place similar to heaven. There are billions of spirits in this world (spirit of plants, of leaves, of trees, of forests, of landscapes, of lakes, rivers, water etc.) so it must be incredibly big.
And because itÃ‚Â´s a spiritual place (obviously) it mustn't be bound to something like a real universe. That would mean that the size of the spirit world is not limited by the size of a planet.
Where is umi's body?
Good question, if they didnÃ‚Â´t show the mandrill in front of the cave, I would have said, Koh swallows his victims or turn them into spirits and makes them a part of himself. But the mandrill ran around freely and because he has no mouth, maybe he will starve... like Ummi??
BTW, I LOVE the idea of Aang meeting kids of his past lifes. That would be so awesome