What happened in "If I Stay"?

I'm going to use this PDF, which contains the text of If I Stay, for referencing.

• On the 11th page of the PDF, it says,

"Pieces of [Mia's] father’s brain are on the asphalt."

• Also on page 11, it says,

"[Mia's mom's] lips are already blue and the whites of her eyes are completely red, like a ghoul from a low-budget monster movie."

• On page 12, it says,

"Someone has put a sheet over my father"

Dad is confirmed dead

• Also on page 12, it says,

"And a fireman is zipping Mom up into a plastic bag...The older [firefighter] explains to the rookie that Mom was probably hit first and killed instantly"

Mom is confirmed dead

• On page 54, we have a whole bunch of paragraphs about

"[Mia's relatives] are avoiding mentioning Mom or Dad or Teddy."

Then she realizes the truth.

"And that’s how I know. Teddy. He’s gone, too."

Teddy is confirmed dead

All events of the film suggest that it is the spirit of Mia walking around the hospital. She's stuck there until decides whether to stay or not. At a certain point, when she almost give up, her heartbeat slows down (the hospital equipment warns). At that moment she was sure she could make the decision.

In the beginning of the car accident her mother dies instantly.

When they get to the hospital you see Mia being rushed into surgery, as well as her father being rushed into it too. While Mia walks with her body after surgery into her room, she looks and sees a man getting pulled into another room, she thinks that it is her father but then a lady runs in and yells George to see if he is conscious, she then starts asking where her father is and the top desk people start talking to each other stating her mom died instantly and her father died on the surgical table.

She later finds out after Adam shows up and tries to get into the ICU that Willow pops out of the elevator and Mia wonders why she isn’t with Teddy and then she goes over to the hospital room he was in and the doctor talking to her grandparents were talking about how he died.

I totally enjoyed the movie and would now like to read the book. It was obvious to ME that it was Mia's spirit that was running around the hospital, thus realizing her mother, father and brother had been killed or died shortly after the car accident. She was in "limbo" during that time, tring to decide if she should stay or just let go. True love prevails and she decides to live. All the flashbacks were her spirit remembering those times. I thought the most touching scene was when Mia's grandfather asked her to stay, but always told her he'd understand if she didn't. I never caught the relationship of Willow. Perhaps she was Mia's aunt? I do wish they would've stretched the story just a little bit instead of it ending as soon as she opened her eyes. Just a short scene, from like a year later. Is Mia attending Juilliard? Is she and Adam living together?