[Ref in PA]

I cannot believe this discussion has gone on for 6 pages.

Read 4-41-4: "The try ends when the throw is successful, when it is certain the throw is unsuccessful, when the thrown ball touches the floor or when the ball becomes dead."

It doesn't matter if it was a try or a throw from behind the arc. What matters is if the referee judges the orignal throw is certain to be unsuccessful. On a throw that originates behind the arc and in touched by a defensive player near the right block (as stated in the OP) you have one of two choices - GT or a throw that is certain will not go through the hoop. If it was not GT then the try has ended by definition. Case 4.41.4 then applies.

To argue 5.2.1 applies over the defintion in 4-41-4 and case play 4.41.4 is irresponsible, you are picking a choosing what you wish to enforce. If you did that in my area, say goodbye to varsity and post season refereeing.

Cases repeated here again for reference.
4.41.4 SITUATION B: A1's three-point try is short and below ring level when it hits the shoulder of: (a) A2; or (b) B1 and rebounds to the backboard and through the basket. RULING: The three-point try ended when it was obviously short and below the ring. However, since a live ball went through the basket, two points are scored in both (a) and (b). (5-1)

5.2.1 SITUATION C: A1 throws the ball from behind the three-point line. The ball is legally touched by: (a) B1 who is in the three-point area; (b) B1 who is in the two-point area; (c) A2 who is in the three-point area; or (d) A2 who is in the two-point area. The ball continues in flight and goes through A's basket. RULING: In (a) and (b), three points are scored since the legal touching was by the defense and the ball was thrown from behind the three-point line. In (c), score three points since the legal touch by a teammate occurred behind the three-point line. In (d), score two points since the legal touch by a teammate occurred in the two-point area.