A possible scenario.
The first shot rings out startling the pigeons on the TSBD roof. This is spotted by Baker. By the second shot Baker is racing down Houston. By the third shot he is closing in on the TSBD. Seconds later he dismounts and runs to the main entrance where he hooks up with Truly. They enter the building, ahead is a set of double doors and beyond this a single door. Once they are through this door they are in a large open storage space. They race towards the Domino room where they will turn left around the stacked boxes to get to the elevators.
Point 1 - Once they are at the corner where the door leading into the Domino room is it is impossible for Adams and Styles to get past them unnoticed. Roughly 30 seconds has passed since the last shot.
On the fourth floor Adams, Styles, Garner and Dorman watch the motorcade go by. Shots ring out. The ladies watch as the Presidential limo heads for the Triple Overpass. A moment of stunned silence which is broken by Adams suddenly announcing she wants to go outside to see what is happening. Adams and Styles race out of the office door heading for the stairs in the north-west corner. Garner follows them at a more sedate pace, as she reaches the office door the younger women are already on the stairs.
Point 2 - By the time Adams reach the stairs Baker and Truly are already racing through the first floor storage space. They must cross paths but they don't. How can this be?
As far as I can see there is only one possible, logical explanation for this.
Baker and Truly abandon the elevators and race up the stairs. On the second floor Truly turns sharp left for the stairs leading up to the third floor. Baker is following close behind but spots movement through a door on the far wall. He immediately heads for this door, once through this there are double doors to his right and the door to the second floor lunchroom straight ahead. Baker enters the lunchroom to confront Oswald, Truly, realising Baker has gone AWOL, follows him into the lunchroom where he explains Oswald is a TSBD employee.
Point 3 - This is the only possible moment that Adams and Styles can get past Baker and Truly without being noticed. The men in the lunchroom, with two sets of doors between them and the stairs, are oblivious to the young women racing down the stairs who are equally oblivious to the presence of the men in the lunchroom.
Baker and Truly finish with Oswald and continue on their way up the stairs. Garner makes her way towards the stairs. The stairs are not enclosed, anyone coming up or going down must step onto a large open space. Moments after the girls disappear down the stairs Truly and Baker appear on their way up. At no point does Oswald appear on the fourth floor.
Oswald finishes his lunch in the Domino room and decides to get a Coke. He makes his way to the stairs in his usual slow ambling way, probably unaware of the shots ringing out on the sixth floor, up to the second floor and into the lunchroom. He is making his way to the Coke machine when the door bursts open behind him and he has his confrontation with Baker and Truly. Totally unfazed by this encounter he gets his coke and leaves the lunchroom, though the double doors then sharp left through the door into the second floor office space where he encounters a flustered Reid who says something to him to which he mumbles a reply. He walks through the office space, down the corridor and takes the stairs leading to the front entrance.
