OK David on the roll call stuff...Bugliosi states that Oswald was the only one who had left the building therefore implicating his guilt.
If Oswald was in such an all-fired hurry to get out of there, why did he [supposedly] stop off at the soda machine and grab something to drink?
Perhaps to avoid the men who he heard coming up the stairs from the first floor to the second floor, or at the very least, have a reason to be there (pretend to be thirsty, wanted a soda) since it became obvious to him that he probably wasn't going to be able to avoid these men, whoever they may be (Truly and Baker).
You really don't understand the very simple concept of one beginning to make his way down from the second floor to the first floor and then, upon hearing men coming up those very steps, deciding to enter the vestibule leading to the lunch room pretending as if that was his destination all along (versus going down to the first floor on his way to leaving the building)?