Re: Wot, no Avatar?
There's a pretty strong sic-fi element in the Avatars themselves. The ability to put your consciousness in to another body, cloned or otherwise, could be incredibly relevant in the near future. Would full-body replacements be available to the public if they could afford them? Would it be possible to transfer minds completely from body to body? Is immortality the next step for the super-rich?
As for the difference between the Na'vi and the other creatures in the area, just look at the huge difference between humans and other various animal and insect species on Earth. It's unlikely that the Na'vi are related to the animals of Pandora for the reasons you list. However, it's not impossible.