Ok, ill start with a A Link to the Snitch. His main story plot was to save the children, and then save the World from the twili NONE were possible without Midna, saving Zelda wasnt part of the plot, he only met zelda cos MIDNA told him to go there, so she could help them both.
But while reading, maybe your refering normal zelda by your first point, ok, your right there. But the plot of TP is not "Link has to rescue Midna" (guessing you have never played TP). As is said, its saving the children, then saving the world (Midna infact saves Link), However, Midna story plot was to save the twili realm, then save the light realm, Zelda and Link. They both needed each other to complete there own quests, both as importnat as the other.
Even those that do class Midna as a "supporting character" she does far more than a normal one and should be included in brawl.
Now Luthien, you have listed the parts before Link meets Midna, however, if you watch the story plot, Midna says what happens to her before she met link. Midna had Zant and Ganondorf curse and remove her power, she already knew Zelda and has done so much before meeting Link. Link is really only doing basic learning of controls (so you can paly the game) and setting up the story plot (then being attacked so he can go in the twili and meet Midna) for the start of the game, while midna has already done everything that involves the story plot. Link is only important because he was so pure, he turned into a wolf instead of a ghost in the twili, Midna relises that he can help her as he is a phyical being who is the only person in the twili who wants to return it to normal, Link is at the start being used for Midnas needs.