flip a coin
By definition, a ball "in the air" is a loose ball, and therefore no one has possession. However, depending upon the circumstances, it's possible that only one team has the right to gain possession (for example, a punted ball).
Yes. According to the (new) NFL rules a receiver must maintain possession of the ball all the way to the ground.
In the NFL, if you bobble once and drop the ball before retaining possession, it is an incomplete pass. However, if you bobble, gain control over the football and take two steps before dropping the ball, it is ruled a fumble, and the ball is available for either team to recover and gain possession.
Answer Yes.
It signals a change in possession of the ball.
The defense will have the ball at their own 20 yard line.
It depends on whether or not he has the ball in his hands when he hits the ground of the endzone. If it is not in his possession, then it is a fumble and can be recovered by a defender. If it is in his hands and he touches the endzone, then it is a touchdown. If he was in possession of the ball when the ball crossed the goal line, it's a touchdown. As soon as the ball breaks the plane of the goal line, the ball is dead and the play is over. Anything that happens after that is irrelevant.
Yes. In both college and the NFL, if a field goal attempt does not cross the line of scrimmage and is recovered by the kicking team, they will continue with possession of the ball. This ruling doesn't really come into play on 4th down kicks because the kicking team will lose possession of the ball on downs. But on third down, if a field goal attempt is blocked and the ball does not cross the line of scrimmage, should the kicking team recover the ball they keep possession and it will be fourth down. If the ball crosses the line of scrimmage, possession goes to the defense.
A 'muff' is generally used with punting and occurs when the player that is catching the punt drops it or 'muffs it' without ever having possession of the ball. There is a distinct difference in the NFL rules between a muff and a fumble. If a punt receiver fumbles a punt, that means he had possession of the ball and then fumbled. The punting team can recover a fumble and advance it as many yards as they can get. If a punt receiver muffs a punt, that means the receiver did not have possession of the ball. The punting team may recover a muff but may not advance the ball. The punting team would gain possession of the ball at the spot of the recovery.
Possession means to have the ball.
No. The NFL's definition of a touchdown is: Touchdown:When any part of the ball, legally in possession of a player inbounds, breaks the plane of the opponent's goal line, provided it is not a touchback.