1. When a player challenges a call, using hawkeye, does he still have a certain amount of time to change his mind or wave off the challenge? Can he wave it off immediately or can he wait until the video starts playing on screen then wave it off?
I have seen a player challenge then immediately take it back and that was allowed. Don't know if there is a specific time deadline for cancelling a challenge though. Probably up to the umpire to decide.
2. When a ball is hit way out of bounds (i) into the stands, are the spectators obligated to throw it back or can they keep the ball? or if (ii) from an outer court if a player hits it over to another court, does a ballboy have to look for it or do they keep playing with one less ball?
Before, the spectators were indeed obligated to throw back the ball to the ballkids. But I heard the rules have been changed on that and they are now allowed to keep it. And yes, the play continues with one less ball.