Steve, here is a frame from Dorman’s film that shows Tina Towner (circled in yellow) standing and looking down Elm Street in the direction of the Limo. (Edit: Tina’s hair appears to be flying outwards in all directions as if she had just spun her head around, perhaps from the TSBD to the limo.) This Dorman frame was taken just before Z313. It appears to me that Tina must have been pulled down immediately after the first shot and perhaps held down for a few seconds but had gotten back up by the time of the third shot at Z313. I can imagine that being held down for only a couple of seconds might seem like an eternity to a 13-year old under those circumstances.

By the way, the crouching figure circled in green appears (to me) to be Brennan looking up towards the sniper’s nest.
Towner said she was pulled to the ground after Shot #1 was fired. Her film shows the JFK Limo having passed the sidewalk that runs down from the (E) Pergola Shelter. If her story is true, it rules out Shot #1 being fired extremely early. Her story/film also disproves the 11+ seconds elapsed firing time for the 3 shots from the sniper's nest.