Isn't unobtanium just another word for supernatural? It comes back to supernatural being the easiest explanation.
Yes. This being a fantasy setting, I'm assuming that the unobtainium is a magical substance rather than the product of fictional technology.
You could, in theory, have something similar to dragons without unobtanium if they not only had hollow bones but bones and air sacs filled with hydrogen to give them buoyancy. They can bleed off some of that hydrogen which, along with some other substance, ignites in order to create fire. You'd probably also have to get rid of the front legs though, because the muscles and anchor points needed for the wings would interfere with the front legs.
Yes, this is the Peter Dickinson/"Flight of Dragons" model discussed upthread. It has interesting ramifications, but the resulting dragon is very different from what we're familiar with.
Air is roughly 0.1% the density of flesh. That means, for every cubic centimeter of flesh you want to lift, you need 1,000 cubic centimeters* of hydrogen. A few sacs here and there won't cut it; the dragon's entire body must be a blimp, with a hydrogen cavity enclosed by the thinnest possible layer of flesh. Interestingly, this means the square-cube law is now working in reverse. The dragon's flight now becomes
more efficient as the dragon gets bigger (the hydrogen cavity's lift grows as the cube of the dragon's length, but the weight of the fleshy shell around it grows as the square). The only limit on the dragon's size is when its heart can no longer keep that huge shell of tissue supplied with blood.
This means a dragon which is absolutely gigantic, but also very fragile. Any wound means air gets into the cavity and mixes with the hydrogen, which a) reduces the dragon's ability to fly and b) means it now contains an explosive mix of hydrogen and oxygen. In a battle, it could very easily ignite itself with its own breath weapon. Flaming arrows would be utterly lethal, both to the dragon and to anyone nearby.
It also means the dragon would fly like a fish rather than a bird, literally swimming through the air. It would probably dispense with limbs altogether, except maybe some vestigial fins to help it steer. It would not move quickly, and it would be largely at the mercy of wind and weather. Watching its approach would be eerie; a gargantuan levitating serpent coiling lazily through the sky, very like the Chinese portrayal and not at all like Western dragons.
*Actually slightly more, since hydrogen is 7% the density of air rather than 0%.