Honestly, I found the name "Unobtainium" no less plausible than a number of the heavier elements in the periodic table. Comparing "Unobtainium" to, say... "Promethium", "Ununoctium", "Berkeluim", "Californium"; not to mention a number of the proposed names for the current temporary ones... How does "Unobtainium" defy logic any more than those do?
Or are you claiming that because we haven't discovered it yet, it doesn't exist? In a science fiction movie. Really? Really?
As for orbital bombardment... That's probably the part the Cameron didn't show. Humans go down to the surface and try to strip-mine politely because of politics. Think of the PR win that it would be for the (then current) administration if they can convince these primitive creatures to live/act like humans! But now that we're kicked off planet, time to warm up the nukes.