A1 is inbounding ball after team B scores. While throwing the ball inbounds, the ball hits the bottom of the backboard with A2 recovering the ball and dribbling to their frontcourt. Is the ball hitting the bottom of the backboard during a throw-in a violation.

No violation.

Good call Barney, no violation. As long as it doesn't hit the back or any supports your fine.

Bottom of the Backboard is inbounds. In most gyms it will be a pad. That can allow this to happen a lot more. Good call Barney.