The penalty enforcement cannot result in the ball moving closer than half the distance to the goal. Is DPI the only exception?
Yes, and no. DPI on a regular scrimmage down when the ball is snapped on/between the B-2 and goal are 1/2 the distance. DPI during a try from on/inside the B-3 are 1/2 the distance.
Otherwise, S/2/15 (I always say "It'S 2:15). Spot, 2, or full 15 yards, whichever comes first.