The "Wiegman's run" segment last 27s and as he stops filming the Hesters, Couch and Darnell start.
Myers says 3.6s before Z313 for start of Wiegman but there's a chance he's off by one second at least(but not 4-5).
Let's call it 2.5s before Z313, runtime of 27s means Couch and Darnell start at 24-25s after Z313.
Okay now Calvery's run.
She could probably do it in around 12s but @6s after Z313 in Wiegman she's still in the same general area on Elm(her group had stepped back and pulled east but no real sign of anyone running away yet). So start a 6s and give her another 12s to reach somewhere west of the landing where she or the other woman could be heard by not just Lovelady and Shelley but officers Smith and Baker.
So, if I'm correct, the theory is that @18s after Z313 some combination of Lovelady, Shelley and Smith react to two women, at least one of whom was screaming and they approach her.
Smith claimed the woman said, "they're shooting at him from the bushes", not heard by either of the two Bill's, he is seen in Couch well ahead of BS/BL stalking his way to Elm St proper. So Smith could have talked to her alone, the two Bill's come next, probably overheard some of the conversation, they then follow Smith.
How long for that conversation, 5s?
So, now we're getting close, Couch starts @ 24-25s and the event thus far has taken 23s but as we know Smith and the two Bills are seconds away from Calvery, who is now on the steps for the start of Darnell(and therefor Couch). If she repeats the story for the two Bills it doesn't work, there's no time and she hardly has had time to tell those on the steps by the time Couch and Darnell react to Baker and start filming.
This is in no way an attempt to knock the story as a whole, I think it's golden, here's my attempt to get Frazier involved.
Women run from the horror toward a little safety at the TSBD where they work, Smith hear's them coming and heads directly for them, he meets them east of the steps and out of sight of BWF but not BL/BS, the latter two men head for the meeting and overhear part of the converstaion and the urgency of it, Smith heads for Elm, hand on holster, Shelly and Lovelady follow. Seconds later the two women arrive on the steps and tell those there what has happened. "Action!". In Darnell we see BWF attempting to hear them for the first time, where as others who could see the cop run up to the women and probably see one of them point that same cop to the west, have not only all tutned to look in that direction but some like Evans have already ran for the light pole to check it out.
Think about it, Why is Evans already at the lampost and Frazier still trying to work it all out?
Frazier likes to claim he had the best spot in the whole plaza, not for this event he didn't. Positioning was the key for an early reaction to Calvery, that and/or a little added awareness.