A Rock Solid Alibi.....

[Alan Ford]

How realistic is it that he went from the top of the steps to the second floor lunchroom before Truly and Baker got there, and no one saw him?

He went up to the second-floor lunchroom to buy a coke BEFORE the P. Parade

[Charles Collins]

He went up to the second-floor lunchroom to buy a coke BEFORE the P. Parade

He was stopped by Baker and Truly in the second floor lunchroom immediately after the motorcade passed by the TSBD.

[Bill Chapman]

Could it be they were all focused on other things and simply didn't observe him? Many people simply do not notice things or people that are close to them and many also simply do not recollect information.
_Correct.
  AKA 'inattentional blindness'

Inattentional 'blindness' might have been a factor if indeed Oswald was outside (but not noticed) as some here are arguing
The same kind of argument may be applied re inattentional deafness and the stairs

In any case:


billchapman

[Martin Weidmann]

He was stopped by Baker and Truly in the second floor lunchroom immediately after the motorcade passed by the TSBD.

So what? What does that tell you about when he got to the lunchroom?

[Alan Ford]

He was stopped by Baker and Truly in the second floor lunchroom immediately after the motorcade passed by the TSBD.

Not according to Officer Baker's same-day affidavit. But by all means continue in your happily subcritical Warren Gullibility, Mr Collins! 

[Martin Weidmann]

Inattentional 'blindness' might have been a factor if indeed Oswald was outside (but not noticed) as some here are arguing
The same kind of argument may be applied re inattentional deafness and the stairs

In any case:


billchapman

Except that it was impossible to be quiet on those old wooden stairs. Dorothy Garner told Barry Ernest that she knew the girls had gone down (before she saw Truly and the police man come up) because she heard them on the stairs. There is no way that anybody could have run down those stairs without making a noise.

[Charles Collins]

Not according to Officer Baker's same-day affidavit. But by all means continue in your happily subcritical Warren Gullibility, Mr Collins! 

Roy Truly’s same-day affidavit includes the correct floor and room. He was with Baker and knew the building well. But by all means, continue in your nutty fantasy Mr. Ford.

[Bill Chapman]

Except that it was impossible to be quiet on those old wooden stairs. Dorothy Garner told Barry Ernest that she knew the girls had gone down (before she saw Truly and the police man come up) because she heard them on the stairs. There is no way that anybody could have run down those stairs without making a noise.

The noise can be minimized by someone light on his feet (including his 131lbs) and in good shape
The girls had no reason to worry about being heard.