First off, I never realized that Rocks had Hydrogen in them!
All that about dragons producing hydrogen is pretty good, but all guess-work. I doubt it was really that way with dragons halfway floating. It would explain the balrogs fire though.
As for Smaug's glow.... I really have no idea. If it was hydrogen, it would probably be described more as a flame than a glow.
The elves on the other hand, were immortal, so it would make sense that starving couldn't kill them. But they had physical bodies that needed sustainance. So basically, they would not die from starvation, but they would be trapped in a nearly lifeless body. I don't know that that made much sense, but... that's all I can think of now.