Was Simba's backstory (pp. 126-127) based on anyone in real life?
Not one bit.

On the way to Doug's house, Scott is distracted by two billboards. Did those really exist?
The classy bank display with the perfectly-chosen redhead is a fictional item, but the billboard with the four scantily-clad women surrounding a giant beeper is pathetically real. Most Angelinos are painfully familiar with the crass displays of JJ the Beeper King.

Scott has an uncharacteristically strong reaction to Harmony's photo. Is this "love at first sight" or something else?
Something else. In the beginning, Scott is attracted to Harmony's media potential. Then when he gets to know her, he's enamored by her simple charm and authentic nature. It's not until she becomes a huge media sensation (in Chapter 14) that Scott begins to have non-platonic thoughts, and even then he admits that his attraction to her is bolstered by her new celebrity power.

What's the deal with the photos of the little boy on Doug's dresser (p. 132)? Is he alive or what?
Scott never asks Doug about it, and I never bothered to think up a scenario. Let's assume the kid is Doug's son, and is now living with Doug's ex-wife in Hackensack, New Jersey. Hey, not all backstories are interesting.

Was Simba really trying to "charm some extra allegiance" out of Scott in the car? Or was he simply overreactiing?
He was definitely overreacting. Keep in mind that Scott lives in a world of hidden angles and ulterior motives. As a result, he tends to see subtle PR campaigns everywhere he looks, even when there aren't any.

Where did you come up with Harmony's tragic childhood?
In researching Harmony, I came across a non-fiction book by Miles Corwin called And Still We Rise, which chronicles the struggles of a group of gifted black high school students in LA.'s Crenshaw district. Some of the girls featured in the book had absolutely heartbreaking childhoods. I basically threw many of their stories into a blender, added a few of my own touches, and then mixed it all up into Harmony's past.

Scott mentions "the brief and forgettable career" of the 80s funk band Picadilly. I don't remember them at all. Did they really exist?
Nope.

What's the significance of Harmony's brain damage?
You know, I get this question a lot and I never know how to answer it. In writing, there are conscious decisions and unconscious decisions. The brain damage thing was definitely an unconscious decision. It just felt right for the story. For a more detailed answer, you'd have to drag me to a hypnotist.