Question about defensive alignments

If the offense is in a formation like a diamond spread, where the center is uncovered. Can the defense play head up on
the center?
Yes, they can line up on him. The restriction on lining up over him only applies to scrimmage kick formations. However, they may not initiate contact with the snapper until 1 second has gone by. This applies any time the offense does not have a player directly under center.