Good discussion.
As LZ says, impetus is from the Team A snap, and the ball becomes dead behind A’s goal line. So, the pure result of the down is a safety - potentially, 2 points for B.
The illegal batting of the ball (a foul) does not change the impetus, because the ball did not leave the end zone after Team A put the ball there. (More about that later.)
Team A has two options: 1) Decline the penalty, and allow a Team B to get 2 points by the safety, or 2) accept the penalty for the illegally batting of the ball foul, which is a rare 10-yard 3&1 principle/Basic Spot enforcement foul. The Basic Spot in this case is the Previous Spot (related run ended behind neutral zone). Thus, the penalty is enforced at the Basic Spot (A-5). The penalty will leave the ball at the line-to-gain (A-15), resulting in a first down. Due to the ‘apparent’ safety, the game clock will start on the snap, and the play clock will be set to 25 seconds and will start on the Referee’s signal.
There might be a scenario in which Team A might decline the penalty, but this isn’t one of those. Team A will accept the penalty.
So, now for more on impetus.
Prior to 2013, it was a true statement that “impetus could not be changed in the end zone,” supported by 2012-8-7-2-b-1 “(Exception: The original impetus is not changed when a loose ball is batted or kicked in the end zone.”
Until then, John Adams had interpreted that a loose ball traveling from an end zone back into the field of play, then, with no change in impetus, returning behind the goal line where it becomes dead, is the responsibility of the team that put the ball in the end zone originally. So, under those circumstances, if team A carried, fumbled or backwardly passed the ball into their own end zone, the result would be a safety. If, under those circumstances, Team A fumbled, or backwardly passed the ball into B’s end zone, the result would be a touchback.
Bottom line at that time: Impetus was the action with/to the ball IN THE FIELD OF PLAY, that caused the ball to be behind a goal line, where it became dead. An action with/to the ball in the end zone did not change the impetus. At the time, a loose ball already behind a goal line that, by whatever means, happened to travel back into the field of play was still the responsibility of the team that last imparted impetus to the ball that put it from the field of play into the end zone. Unless somebody gained player possession, or batted or kicked the ball in the field of play, that original impetus did not change. So, if it then crossed back into the end zone and became dead, the ruling was dependent upon who put it there originally.
But then Redding decided to change that.
In 2013, he eliminated the language mentioned, and interpreted that, if somebody batted or kicked the loose ball while it was in the end zone, and the ball travelled back into the field of play, such batting or kicking in the end zone imparted new impetus. So, if the still loose ball then re-entered the end zone and became dead, the action imparting impetus in the end zone is now responsible.
That’s where we stand today.
I understand the rule, and will enforce it as interpreted. But, I will go on record as saying I think this was a bad change, and I simply don’t like it. Adams’ interpretation was much simpler and cleaner. Impetus was what put the ball from the field of play into an end zone where it became dead.