Just looked back on YouTube at a few similar plays and am convinced that, IF EARLY, the play would’ve resulted in it being Arizona ball at the six yardline and NOT a touchback:
2008 Eagles/Cowboys: DeSean Jackson flings the ball *backwards* before he scores. Nobody picks it up and ball is dead at the one. Refs initially rule TD so play is dead anyway (I believe this is before you could challenge a “clear recovery” so even if Dallas picked it up, the whistle already blew and so the only ruling is that it is Philly ball at the one). Eagles scored on the next play.
2006 Raiders/Chargers: Vincent Jackson catches the ball going to the ground for a first down, is never touched, gets up and spins the ball *FORWARD* intentionally. Raiders pounce on the loose ball. Ruling is an illegal forward pass, five yards from the spot. Chargers keep possession through this loophole.
The 2006 play would suggest that if the forward motion is *intentional*, it is considered a forward pass, and if incomplete, the play is dead. Illegal forward passes can be intercepted and returned only if caught midair. They are NOT treated as a fumble, and so Murphy’s play would not have been a touchback. Just a five-yard penalty from the spot of the “pass,” and then Arizona ball first and goal from the six. We probably still win.