What's the deal with Scott and his ex-lover Gracie? Were they married or what?
Scott explains later in the book (pp. 257-258) that they did indeed have a wedding, but they never legalized the marriage. We thought we were being cool in our post-modern detachment, he tells the reader.

In her anecdote, Miranda tells Scott that 3,828 people died in the 1984 Union Carbide accident in Bhopal, India. I thought the death toll was a lot higher.
That's the figure the State Government of Madhya Pradesh came up with when they finished cataloguing the damage of that inconceivable tragedy. But there were over two hundred thousand others injured from the poison gas leak, so you may be thinking of that.

Is there really a book by Bruno Bettelheim that provides a Freudian analysis of classic fairy tales?
Heavens, yes. I read The Uses of Enchantment in college, and I've been blushing at Jack's beanstalk ever since.

Is there really a Melrose High School?
For the sake of me and my lawyer, I sure as hell hope not.

Was the Annabelle Shane school shooting based on a real event?
Not a whit.

Scott briefly refers to Sean Combs by his old name, Puff Daddy, even though it's P. Diddy now. Was this done to illustrate Scott's rap ignorance? Or did you screw up?
Neither. Keep in mind that this story takes place in February 2001, just weeks before Sean Combs officially went from "Puff Daddy" to "P. Diddy" (and just days before he and J.Lo went belly up). So at the time, Scott was right.

Both Hunta and Mean World Records are entirely fictional, right?
100%.

Were the "Bitch Fiends" based on some real life incident?
They were loosely--VERY loosely--based on the Spur Posse, that scurrilious clique of high school jerks who gained wide media attention in the early 1990s for their sexual exploits. I'd forgotten all about those charming lads until I read Susan Faludi's Stiffed.