So, Lord Ghirahim can be hurt by Phi's (the Master Sword's) energy? Also, didn't LG say something about the sword being the only reason Link's alive -- during the demo in which we see Link and him duel?
We saw that Ghirahim just got tired when Link hit him with the Skyward Sword, so it's more than probable...
Um, I think him getting tired, is just for you to get a few hits in. I mean, the only way (as I know) to make him tired, is to Shield Bash him when he does a running slash. So I think that's how he gets tired. In my opinion.
I think it's a dream. Like Zelda being eaten and Phi coming to "save" Link. They may be visions of the future, that's why it's inconsistent.
But as the 'whale' eating Zelda is probably just a symbolic representation for Ghirahim or whatever evil is after her, I doubt Link would actually see Ghirahim in his dreams.
Not to mention fight him in his green garb, which he gets much later on. I just don't think his dream is this clear.
Hmmmmm...it is very possible but only Link can wield the Master Sword so that means only Link can wield the Skyward Sword.