Knowing fully well that I will get admonished and told to keep my nose out things on the NFHS forum, I am going comment, anyway.
In the video example provided, how does "injury" become a factor, whatsoever? If we don't interrupt the game for an injury, then any pain/discomfort that a player may be suffering is of no consequence to us as officials. The player in question (wearing an eligible number) appeared to be trying to leave the field, under his own power, and, until the last moment, was moving to to leave the field in a reasonably direct and expeditious manner. While he is moving toward the sideline, a substitute (also wearing an eligible number) for that team runs onto the field from their sideline, 'apparently' to replace the player that appeared to be leaving the field. There was no reason to interrupt the game for the guy that appeared to be leaving the field - even if we had a player count for team A, and we know he is one of 11. For all we know, they will send in another sub for him when he gets off the field.
So, then, when this guy gets near the sideline, he stops - you hear coaches telling him to "stay right there" - the ball gets snapped, and this guy, unopposed by an opponent, runs downfield to catch an apparent touchdown pass.
This is a deliberate use of a "hideout" play, and is patently unsportsmanlike conduct. Not anything for which we would interrupt the game for an "injury," fake or real. Live-ball foul, 15 yards from the previous spot, repeat the down.
As to whom to assign the foul, that may be different for NFHS than other rule sets. In NCAA, this gets assigned to the "hideout," player (as much as we might want to assign it to a coach). If I'm a player, and I already have one UNS, and my coach tells me to run that play, I'm gonna be p-----, uh, annoyed.
Cheating. Pure and simple.