Spotting the ball

[bsaucer]

I'm a fan, but I sometimes like to "nit pick". I have questions about spotting the football.

1. As I understand it, the forward point of the ball marks the line of scrimmage. Is that correct?

2. If team A downs the ball so that the rear end of the ball breaks team A's goal plane, is that a safety, or does the entire ball have to be behind the goal line?

3. When the ball goes "over on downs", is the line of scrimmage moved to the other end of the ball, or is the ball moved to the other side of the line of scrimmage?

4. If the ball is down with the ends pointing toward the sidelines, how is it spotted for the next play? If the ball is turned 90 degrees, can that result in a touchdown, first down, or a safety?