Safety or Penalty
 

This actually happened last Friday night in a playoff game
B intercepts a pass at their own 6 ~ he circles back into his own end zone ~ A attempts a tackle. B is standing with both feet in the end zone - reaches out so that the ball is completely outside the goal line plain and throws an illegal forward pass.
Ruling??

Illegal forward pass. Spot of foul is B's EZ, so it results in a safety.

What makes the end zone the spot of the foul if the pass originated from the field of play? "...reaches out so that the ball is completely outside the goal line [plane] and then throws an illegal forward pass." Were you assuming that between the "reaches out" and the "then", he cocked the ball by bringing part of it back behind the goal line to throw it? That could be, but since the original poster didn't say it, I wouldn't assume it. Still, I think the point of origin of the pass is taken to be the release point, not the start of the passing motion while the ball is still in hand.

He's in the end zone. If a player throws an illegal pass, with his feet on the 10 yd line, but the ball is in front of him at the 11 yd line, where are you penalizing from, the 10 or the 11?

Under the NFHS code, NF:7-5-1 designates the "spot of the pass" as the position of both feet "when the ball is released".

But that's not the way the spot of the pass is determined. It's the criterion only for determining whether a legal or illegal forward pass has been made. 7-5-1 doesn't define spots.

For an illegal forward pass, the spot is going to be determined by 2-41-9b., "Where the runner loses possession if his run is followed by a loose ball"; also 10-3-3b, same exact wording.

The 11, same as for any other run that ends in the ball's becoming loose.

Can you think of any example where an enforcement spot is determined by foot position? For contact fouls presumably some body part is meant -- presumably the part that made contact with an opponent in the way specified -- but not the feet unless the foul is kicking an opponent!

If he looks like he's in the end zone, I'm ruling him to be in the end zone. If I happen to be calling a game where someone can do a frame-by-frame look and tell the ball was released from outside the end zone, good for them. I'm not seeing a reason, when every spectator is going to see a penalty committed from the end zone, and when I'm probably not in position on the goal line to be certain, or even suspect otherwise, that it shouldn't be ruled from in the end zone.
So, letter of the law, penalize him three inches. Practical application of the law, award a Safety. Kid shouldn't be throwing a forward pass, anyways. He'll learn a valuable life lesson-don't do stupid things in public-and go on to live another day.

How do you know this is what the official saw? Seems exceedingly unlikely, given a normal throwing motion where the ball is above and slightly behind the player's head.

That may be a normal throwing motion for intended forward passes. However, what's the likeliest scenario here, given that we have no other details about the play? That he was playing "razzle dazzle" & thought he could hit a teammate downfield? More likely he was trying to pass to a teammate across the field, intending a backward pass, in which case it's likelier the ball was brought in front of the passer for a throw across the body.

Regardless, the end of the run is where the passer gave up possession. Maybe that's where the ball left his throwing hand, or maybe you could rule that he lost possession at some earlier time in his arm motion. Depends what you consider "held or controlled".