I know that the building looks quite large in all the photos, but it really isn’t. And it was not much of an obstacle course, as the boxes had to be stacked neatly to keep track of which books were in which boxes.
Getting to the stairwell is rather simple, and stashing the rifle under a box could have been done while barely breaking stride. Its not like it was craftily hidden.
The stairs themselves are hardly intimidating in size, and navigating them in 90 seconds is reasonable. Remember, the NFL gets off three running plays for no gain in 90 seconds, complete with the huddles. Not to mention the figure of 90 seconds itself is only an estimate.
I wouldn’t expect that he would even speak with the co-workers in the break room, as he was apparently not “best buds” with any of them. Timing Oswald’s movements inside of the building, post shooting, is not anthing that can be established with exactitude.