1. My brother and I developed this theory on the Kokiris back in 1998 when we first got OoT: They turn into the Stalchildren out in Hyrule field when they leave the forest. I don't know where we got that theory (it's been like, 11 years, and I was just a 4th grader then!!) but I think it partially came from the Kokiri girl during the trading quest that gives you the Poacher's Saw for the Odd Mushroom. She said that those that get lost, turn into Stalfos, or something like that. I know she was most likely talking about Hylians, but since the Kokiri are an enchanted race created by the Deku Tree, maybe when they leave the forest, they turn into monsters too because they are lost? Also, I think the red-haired Kokiri boy near the exit of Kokiri Forest says something about leaving the forest when Child Link leaves...I'll check it the next time I play OoT just to be sure.
2. The Zora Question: I really believe they can be away from water for how ever much they want. Even though you always see them in water, I think they work more like frogs instead of fish. I also believe that Ralis wasn't injured from being away from water, but because Zora's Domain was attacked by the Twilit Messengers in TP. I believe that was the same time his mother was killed. I'm pretty sure Telma said something like that, but it would help him out if he was near water. I could be wrong on that too, but that's what I remember.
3. Is KG really Rauru? I don't think so. To me, it seems like they are completely different sages. I mean, Rauru was all too happy to inform Link about his destiny and his place in this world. I would think that when Rauru was telling Link about his sagehood, he would have mentioned something like, "I knew you were the one when I saw you leave the forest" or something like that. Can't guarantee it, but it seems like that's a running theme with the Zelda games. Zelda revealed herself as Sheik in OoT, King of Red Lions revealed himself as Daphnes Nohansen Hyrule in WW, Midna revealed herself (sorta) at the end of TP that she was the Princess of the Twilight. Even if Rauru didn't mention it when Link first woke in the Sacred Realm, if he was KG, I really believe he would have said something in his last meeting with Link at the Desert Collossus after he claimed the Silver Gauntlets.