I remember reading somewhere someone writing that DP is "an echo chamber from hell"...I guess that is an accurate statement as if there were a shot or shots from the GK area why were the three men at the base of the steps then running back up them TO danger and why were the SS agents in the Queen Mary looking back after the shots? I mean everyone can make a case for anything in that environment that day.
During the HSCA test shots in DP(according to either Gary Mack or Robert Groden) one witness to the event said you just could not tell where they were coming from.
Slightly better than that is Lee Bowers, he told Mark Lane(1966) that during refurbishment work on the TSBD itself some months prior to the assassination, noise from that work often sounded like it came from the GK area(and I think vice versa).
Having watched Buell Frazier a lot recently I know he is also fond of saying "echo chamber" but IMO that's something he has read of in the intervening years and this has helped him explain how he changed his story from the original where said he thought the shots came from the west, to now, "above me" and not just that but "directly above my position". Apparently this is the only time he noticed it...
Not to nitpick on your second point but, one man on the steps doesn't move, the second one only runs as far as the first then stops and the third may have just ran to the wall where most likely a woman stood, who he knew and perhaps was concerned for.
Those agents, all three of them where looking back to somewhere at ground level. That in itself is interesting in that they could not tell it was coming from above and the gun had to have been aimed almost directly at that spot we now see them at in Altgens.
I think it's Ready who was the most alert(keep an eye on him in Hughes and Muchmore on Houston), the agent behind him on the running board may have partly reacted to him since he's not looking in exactly the same direction, the third in the back seat has spun right around and probably also reacted exclusively to a loud noise.
Perhaps those further back in the parade who claimed they could tell it was coming from above just had more time/opportunity to work that out, or simply, saw someone else near the intersection looking up IDK but I always have trouble believeing folks who say they could tell exactly/immeadiatly and there's a few of them
