A quick recap so far:
The Oswald-police officer-Truly encounter at the front entrance initially reported was moved deep into the building, at or near the rear stairs. The third-or-fourth-floor-rear-stairway story was quickly superseded by the lunchroom story. This became: the lunchroom stories—Oswald sitting at a table, Oswald leaning against a counter, Oswald standing by the Coke machine drinking a Coke.
The initial plan was simply to transplant Oswald up to the lunchroom, stretch the timeline and worry about the details later. Oswald was still alive and had every prospect of going to trial, so his damaging ability to describe a police officer and Truly coming in to the building needed to be preempted by a story involving the police officer and Truly coming into the lunchroom.
Truly's inflated time estimate in the very first reference to the second floor lunchroom incident gives us a clue how this would be played out:
FBI Report by Williams-Pinkston, 11.22.1963 (dictated 11.22.1963)
Two or three minutes? Yeah, right. The Warren Commission would struggle mightily to stretch the time to 90 seconds. But the state of the evolution process on the evening of November 22 was to simply to chunk out enough time to allow Oswald to run down from the sixth floor, but not too much time to delay his exit from the TSBD. The details could be finessed later. Then all the Oswald accusers had to do was drive home how very odd it was for a man to be alone and loitering in a lunchroom while everyone else was outside or looking outside.
FBI Teletype from Dallas to Washington, 11.23.1963
Like Ol' Man River, First to Second Evolution just keeps rolling along.