Who were these "18 or 19 others" who left the building right after the shooting? I've never heard about a single person who was in the building who left shortly after the shooting. And certainly not "18 or 19". The only person that I am aware of who was in the building at the time of the shooting and left afterwards - without permission from the police hours later - was Oswald.
The TSBD was not totally sealed off until maybe 5 or 10 minutes before 1....even though they lied and said it was 10 minutes after the shots [ridiculous]
Wesley Frazier left TSBD between 1:00 and 2:00 pm, about the same time Truly
claims he made his roll call [which was another lie]
Charles Givens returned to TSBD after the shooting but was
refused entry by a Dallas policeman. An APB was put out for him [a big question was why?] and he was later interviewed at City Hall by Captain Will Fritz. Truly was aware of Given’s absence, having seen him prior to the assassination. Truly also said he saw Oswald in his encounter with him in the second-floor lunchroom. Using that logic, Oswald should have been exonerated.
There were various others that either left before the shots or were outside..and couldn't get back in and then went home.