I'm sorry - Donnie Darko

Alright so I'll start this off by saying that I'm sorry for making the 1,000th Donnie Darko thread. But before making this I read EVERY thread about it, including the "Everybody dies alone" thread. I saw this movie for the first time last night and just watched it for the 2nd time and I have a ton of questions that were unanswered after reading all the other threads.

The biggest question I have seems really stupid, but I just don't understand. Everyone says that Donnie had to return the engine back to the right time. I just don't understand why he needed to do all that stuff. Say he didn't flood the school, burn down Jim Cunningham's house, met Gretchen, killed Frank, the same exact plane still would have taken off and lost it's engine and it would have still landed right in Donnie's room, killing him, and everything returns to normal. It's hard to explain but if anyone could elaborate on this that would be great.

1. Frank. I'm having the hardest time figuring out who this guy is. I know that he is also the sister's boyfriend, does Donnie know this? I don't understand if the Frank that helps Donnie is the same one that is the sister's boyfriend. What I mean is, are there 2 Franks in 1 parralell universe and none in the other? I don't get it. And how come at the movie theatre Donnie asks the same 2 questions twice?

2. Grandma Death. I don't fully understand what her role is in the movie. Is it only to have written that book so that Donnie can learn about time travel, and so that there is that scene at the very end happens?

3. Cellar Door. Is the only purpose of this so that Donnie will try to go to Grandma Death's house? And then get mugged/Gretchen dies/Frank dies?

4. What is the significance of the bunny suit, if there is any?

5. At the very end, during the "Mad World" montage, when it shows Frank, his eyes are fine, and he touches his right eye. Why are his eyes fine then but not later on?

6. I don't understand the portal at the movie theate. What is the point of it? Just to show the picture of Jim Cunningham's house for Donnie to burn it down?

7. I also don't understand why earlier in the story why earlier, it shows the filing cabinets in the sky? I read earlier that it was a portal, I just don't understand why it is shown/what the purpose is?

8. Is there any significance of the political stuff with Dukakis and the other guy running for president?

9. Is Noah (Science guy) and Drew Barrymore married in the movie? During the Mad World montage it looks like they're sleeping in the same bed.

10. I read in another thread that you can see Frank's car early in the movie, and I just re-watched it and didn't see it anywhere, which scene is it in?

11. Is there any reason why during Jim Cunningham's speech when he talks about a person he knows with problems the guy's name is Frank?

12. Is the reason that Donnie's mom and sister not crying because they were both on the plane, and they somehow know that Donnie died for them? And Gretchen remembers a little so that's why she waves?