The final shot likely came from the bridge...
Cancellare caught this fellow sitting on the other side of the bridge...
Either a police officer... or dressed up to look like one...
I believe Unger colorized the second pic...
The last pic is a rendition to help viewer orientation...
By "bridge" you mean the triple underpass. I don't think the spot you've identified on the triple underpass is a plausible shooting location.
However, I will point out that the first thing that DPD Chief Jesse Curry and Sheriff Bill Decker did after the shots rang out was to order their men to go to the area of the triple underpass and the railroad yard behind the grassy knoll to see what had happened, clearly indicating they believed shots had come from somewhere in that area.
And of course we know from the photographic evidence that large numbers of people in the plaza, including some police officers, rushed toward the grassy knoll after the shots were fired. We also know that J. C. Price saw a man running away from the grassy knoll fence and into the railroad yard right after the shooting. A number of witnesses said they smelled the pungent odor of gun powder on or near the knoll right after the shooting. And, the Wiegman film shows a small cloud of smoke hanging in front of the trees on the knoll, which confirms the eyewitness accounts of seeing gun smoke emitted from a point just behind the fence on the knoll during the shooting.