The, "unobtanium," issue has been addressed before, but I think people are failing to note the genre savvy involved. Cameron isn't claiming to have made the name up anymore than he actually claims to be king of the world. He was making reference to something else. In one case, he wrote his scientists referring to an impossible substance as, "unobtanium," in an attempt to be clever in-world. In the other, he quoted from his own film, the film he'd just won an Oscar for, in an attempt to be clever in the real world, telling us that he was feeling really good at the moment. But then some dickhead made it sound like he was bragging unduly (though when you win that many Oscars for one film, you do actually have the right to brag) and claiming to be better than everyone else.

People just don't give the man any credit.