Meaning: Donnie Darko Explanation?

I've just finished watching the movie Donnie Darko and I thought it was a wonderful film but I didn't understand it, the movie seems like it has a lot of symbolism in it and it's very complex throughout. What is the meaning of Donnie Darko? If you do, you should explain it to me.
I have several questions about the movie:

Why did Donnie have to flood the school?
What is the significance of the fat asian girl in Donnie Darko?
why does she keep walking back and forth to the mailbox and back?
What about the little boy and the feather?
Did anyone get the ending of donnie darko?

My friend thinks every one dreamed about it, about whats going to happen to them in the future.
Do you get what Donnie Darko is about? Can someone please explain this movie, like in extreme detail? I need an explanation! I don't know the significance of some things.

Donnie Darko explained:

DONNIE NEVER TRAVELS THROUGH TIME.
Donnie essentially went back in time to save his girlfriend because he knew what was coming and he sacrificed his own life to save Gretchen, and ultimately the tangent universe.

it's a popular myth is that he does but the jet engine is the only physical thing that travels through time. He does close the Tangent Universe which resets time back to October 2nd but that is not time travel.

It was a predestination paradox. Because he burned down the pervert's house, his mother had to chaperone the girls on their trip. The engine from their plane went through the portal to twenty eight days earlier and crashed into Donnie's room.

At the end of the film a timestorm starts to form as Donnie drives up the mountain, it’s the beginning of the TU collapse and is centred over Donnie’s house which is where the TU began. Donnie constructs a time portal from water and guides the engine through it. This restores the Primary Universe back to October 2nd, the birth of the TU. The last 28 days never happened and Donnie wakes up in his bed laughing. The jet engine Donnie sent through the time portal then falls into his bedroom killing him.

The theatrical release of "Donnie Darko" offers very few answers.

Meaning of the movie? Basically Donnie sees his future before it happens, realizes everyone would be better off without him, so lets the plane crush him when it falls in his room

There is a bit of controversy about the Chinese girls role in the film. Some say she is donnies guardian angle and some say it's to show racism and others believe she has no significant meaning. But watching the movie multiply times I feel as though she is used to compare modern day perfection and imperfection. It is toward the end of the movie where she does her dance dressed as an angle and is juxtaposed right after by the skinny stylish good-looking entertaining dance team. Making the viewer feel sympathetic towered her. And then also towards the end when Donnie speaks to her and she drops her stuff revealing she has feelings for him it, to me, shows her aiding Donnie to realize just like her he is an outcast and is imperfect. Where as Jim Cunnigham is classified as perfect but later is reviled he is also an outcast.

Feel free to ask any more questions. This is all just opinion however.