The side is in and the top edge. What about the bottom of the backboard?

Also in play. The pad is simply for the safety of the players.

thats what I thought. I got it correct despite the few loud mouths in the stands.

Legal.
If from a throw-in under the basket, the ball hits the bottom of the backboard and goes onto the court, it has only hit the bottom and it is legal.
If the ball hits the backboard and returns toward the thrower-in, it has hit the back, and is a violation.
IMO :)

Blackhawk

You'll find it here
 

NFSHSA case book...Page 64...9.2.2 Situation A (b,c).