I actually think that the Mickey, Donald, and Goofy we're seeing are the same ones from the Realm of Light; it's just taking place during a different time period. T3M movie has Queen Minnie as a princess. This world also portrays Minnie as one, meaning this is taking place before Mickey became King. Therefore, Sora is actually talking to Mickey from the past. Why in the past? Because the game is taking place in the world's dreams. And what else can a world dream about other than what it remembers prior to being destroyed? In other words, a world's dream should consist of that world's history. If this is true, then Mickey knowing about the Keyblades and about other worlds and, yet, not recognizing Sora makes much more sense. Mickey was an apprentice of Yen Sid before becoming King (Fantasia: Sorcerer's Apprentice reference ftw) and thus, never met Sora, Riku, or the ATV gang.