Two more comments to make to add to the timeline;
1.
The stairs used by Adams and Styles, and allegedly also used by Oswald, were in the back of the building. Some years ago Discovery's "unsolved history" attempted to reconstruct the timeline between the shots and the Baker/Oswald encounter in the 2nd floor lunchroom. Although not a completely accurate reconstruction (they did not include the landings on each floor and used stairs that continued on downwards without interruption) the video IMO does provide us with a fairly reasonable estimate of how much time was roughly needed to get from the front windows of the building to the stairs in the back, which is about 25 seconds.
As Adams and Styles would have to cover nearly the same, if not exactly the same, distance as Oswald, who in the video is presumed to be the gunman, it can be argued that Adams and Styles as well as the gunman would have been at the top of the stairs, about 25 seconds after the last shot, albeit on the 4th and 6th floor respectively.
The video continues to show "Oswald" coming down the stairs (without taking account for the landings) and arriving at the door of the 2nd floor lunchroom some 48 seconds after the shots. If correct, this would mean that "Oswald" would have had to come down 4 floors of stairs (without the landings) in approx 23 seconds. If we add on 2 seconds for each landing, that time time would go up to 31 seconds. In order for Adams and Styles to stay in front of him, they also would have had a maximum of only 31 seconds to come down at least 2 floors.
2.
In order to find out just how much time would be left for Adams and Styles to come down from the 4th floor and arrive at the 1st floor without seeing Truly and Baker, it has to be established just how long it took Baker and Truly to get to the stairs at the back of the building. The excellent 3d animation made by Mark Tyler offers a good starting point for that determination;
Not a valid vimeo URL
It shows that Baker (initially # 27 as motor cop and later # 46 as a physical person) needed approximately 30 seconds after the last shot to park his motor and run to the entrance of the TSBD building. It should be noted that in the animation Baker crosses paths with Billy Lovelady (# 94) who is moving towards the island and we see Gloria Calvery (# 93) running up to Lovelady to tell him the President had been shot, just like he said in his testimony.
So, if the Discovery reconstruction and Mark Tyler's animation are correct, we have Baker meeting up with Truly at the entrance of the TSBD approx 30 seconds after the last shot, which is roughly the same time as we have Adams and Styles at the top of the stairs on the 4th floor and the gunman at the top of the stairs on the 6th floor. There might be a slight discrepancy of a second or so, here or there, but that would not alter the basic elements of the timeline.
If we assume that it took Baker and Truly another 30 seconds to get from the entrance of the building to the stairs in the back, we end up with the conclusion that Adams and Styles would have had roughly 35 seconds to get from the top of the stairs on the 4th floor to the first floor and out of the building. If, as the Discovery reconstruction shows, "Oswald" could have gotten down four floors to the 2nd floor lunchroom in approx 30 seconds, Adams and Styles probably could have gotten down 3 floors in about the same time and miss Baker and Truly by seconds. But the time available to Adams and Styles is probably a little bit more, because if Baker and Truly were indeed at the bottom of the stairs 60 seconds after the shots, they would have taken at least 15 seconds to go up one floor and "meet" Oswald, which seems a bit too much time to me.
The only real requirement for this entire scenario to work is of course that Adams and Styles must have been on the stairs at the same time as "Oswald" was.
What obviously doesn't fit with this scenario is Adams seeing Shelley and Lovelady at the bottom of the stairs. Even more so as both men, if they had been there, at that moment, must have seen Baker and Truly rushing towards the stairs where Adams and Styles had just been on. There is no statement to that effect from either Shelley or Lovelady. For what it is worth, Mark Tyler's animation shows Lovelady arriving at the railroad yard about 60 seconds after the last shot, which is exactly when the WC scenario places him and Shelley near the bottom of the stairs on the first floor. A physical impossibility.