The guys atop the Triple Underpass might have had their vision glued on Elm St as everything unfolded down below them. Your claiming these same guys would have seen a possible "rifle toss" on their left side amidst all hell breaking loose below them is iffy at best.
According to Hoffman the tosser/shooter would have taken much time to walk the full length of the fence before tossing to tossee.
Tosser not seen by Holland nor Bowers (nor Hoffman).
Tosser was supposedly still in that carpark when confronted by Officer Smith(?) a minute or two later. Nope.