Yes, otherwise it is considered an incomplete pass
Any fumble, whether forward or backwards, is a live ball. The offense can recover the fumble, with certain stipulations: If the play is a fourth down, OR if play is within the last two minutes of either half, only the player who fumbled the ball can legally recover for the offense. A forward pass that hits the ground before a player gains possession is a dead ball and the play stops. A backward pass that hits the ground before a player gains possession is a live ball or considered a fumble and the recovering team will gain possession of the ball.
"FF" stands for "Forced Fumble" When a player knocks the football loose or strips the ball from a player that has full possession of the football is considered a Forced Fumble (FF).
Yes. The only 'weird' fumble rule is on 4th down. On 4th down, only the fumbler may advance his own fumble. If any other offensive player gets possession, the ball is dead and placed at the spot of the fumble. Since a kickoff is, by definition, not 4th down, it can be advanced.
A fumble is when a player with the ball drops it.
If the ground causes a fumble the player is ruled down.
A fumble is a fumble, no matter what direction it goes in, but a player cannot advance the ball forward through the air once they have passed the line of scrimmage.
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.
If a rugby player was to fumble the ball and drop it, his team mates would not be very pleased with him.