This floor plan supports Martin's scenario as the NW corner of the building blocks access to the western part of the loading dock,
https://i.servimg.com/u/f39/18/50/79/06/1st_fl10.jpg
No matter how they twist and turn things, the only logical explanation is that Adams and Styles did indeed leave the 4th floor after the last shot, just like they said they did.
Had they stayed four or five minutes later, they would have encountered men coming up the stairs and they probably wouldn't have been able to leave the building. They most certainly wouldn't have made it back in time, from a round trip around the building, to re-enter it before Harkness and two other officers sealed of the back between 12.36 and 12.37.
12.30.00 Last shot
12.30.30 Adams and Styles arrive at the stairs on the 4th floor in the North West corner
Shelley and Lovelady are in front of the building and making their way to the railway yard.
12:31.00 Adams and Styles arrive on the first floor and leave the building through the loading door
At the same time Truly and Baker pass the elevator block on the first floor and run towards the stairs, just missing the women
12.33.30 Adams and Styles arrive at the railway yard and are stopped by a police officer who sent them back to the building
12.35.00 Adams and Styles arrive at the front entrance of the building and Adams overhears a radio report
12.36.00 Adams and Styles re-enter the building just prior to it being closed off by Harkness
When she enters the building Adams sees Shelley and Lovelady, who have just entered the building themselves
12.37.00 Harkness seals off the building
The times are approximations, but the timeline works perfectly and includes all the known information without any witness having to lie.
I challenge anybody to come up with a similar timeline that starts with Adams and Styles leaving the 4th floor 4 to 5 minutes after the shots and still incorporates all the available information correctly.