I talked with a couple of my crew members about this play, and it's really not an easy decision.
As the R, if I saw a wing give an incomplete pass signal (but no whistle), and I saw players pull up as a result of that signal, I'd likely be blowing MY whistle. I'd figure that my wing had dropped his whistle, and the play should have been blown dead.
Upon questioning the wing, I'd find out that:
(a) He ruled incomplete, and either forgot to blow his whistle, or it dropped out of his mouth. The game would be over, and we'd have to live with his (probably erroneous) judgment.
or
(b) He ruled a backward pass, and mistakenly gave the incomplete pass signal instead of a punch-back. Since I would have blown my whistle, we'd have an IW and administer it using the rule in place.