Ok, let's get a few facts straight about Howard Brennan:
Brennan's testimony would have been torn to shreds in a trial. Brennan couldn't even identify which sixth-floor window he supposedly observed, and the Zapruder film shows he was not looking up until after frame 207, over two seconds after the first shot was fired (at around frames 145-160). Brennan said the man he saw in the window was standing when he fired each of the shots, a fanciful proposition that even the Warren Commission rejected.
In addition, Brennan failed to identify Oswald in a police line-up on November 22, even though he had seen Oswald's picture beforehand. Posner deals with this problem by advancing Brennan's claim that he could have identified Oswald in the November 22 line-up but was afraid to do so because he feared Oswald had accomplices who would kill him if he made the identification! Yet, on November 22, Brennan spoke with reporters about the assassination, and he even gave them his name--strange behavior for a man who supposedly feared he would be killed if he identified Oswald in a police station.
Moreover, it was only after months of "questioning" by federal agents that Brennan gave a positive identification of Oswald as the sixth-floor shooter. The HSCA found Brennan's testimony to be so full of contradiction and confusion that it ignored his story entirely.
There is another serious problem with Brennan's testimony that is often overlooked. Brennan said that when he looked up after the presidential limousine had driven away, he still saw Oswald in the sixth-floor window! Brennan added that Oswald remained at the window for at least a few seconds after that! Then, said Brennan, Oswald "simply moved away from the window until he disappeared from my line of vision." "He didn't appear to be rushed," recalled Brennan. Anyone who knows the case well knows the serious problem this poses for the already impossible task of getting Oswald down to the second-floor lunchroom in time to be seen by Officer Baker.
If Brennan had not succumbed to the pressure to identify Oswald, the WC would have treated him much the same way it treated Arnold Rowland. I believe most of Brennan's account, just not his identification of Oswald and the bit about the gunman firing while standing. His statement that the gunman stayed in the window for a bit and did not rush off is consistent with the testimony of Bonnie Williams, who was a few feet below the sixth-floor window and who said he heard no movement in the window after the shots were fired.