First of all, IMHO, the Umpire should not rule on completion/incompletion of a pass unless it is apparent the covering officials (wings, BJ) have no idea. Then if you have positive knowledge, make the call. If the U had hesitated, this probably wouldn't have been an issue. I always defer to the Wings unless they give me that deer in the headlights look.
That all being said, in this scenario, game over. Even if the passer had been ruled down with no clock stoppage, would 8 seconds be enough for the U to get to the ball, place it and R whistle RFP?
Best regards,
Brad