There's some logic to the floating mountains. Explained somewhere, can't be bothered looking for it myself
But it goes along the lines of unobtainium being some super-conductor, explaining its value. And the super-conductive effect can make them repel each other like magnets (or something) so the mountains float.
And still be a normal enough mountain to have internal water-tables so water sources, springs on teh surface, meaning we get waterfalls.
^^Yeah something like that.
Also a question, the Avatar is linked to Jake or who ever is controlling theirs. There must be some direct signal link from the base to the Avatar and back to enable control and force-feedback etc. So some high data rate link going back and forth.
But there's area of Pandora where the electromagnetic field is so strong that the helicopter's guidence and sensor gear simply doesn't work. But the avatar signal has no fault.
And also how can a planet covered in trees and plants . . . not have breathable air!!