Basing anything on 1 single eyewitness does Not make it a Fact. Until the Brennan statement is corroborated, the shots being fired from a seated position as allegedly verified by law enforcement will stand. (No pun intended)
Would the fact that Arnold Rowland also reported that he saw a man STANDING behind a window on the sixth floor be corroboration for Howard Brennan's statement ?
And I would hasten to remind you that no kneeling gunman could have knelt between the window and the stack of boxes behind that window, so a gunman would have had to have knelt behind ( to the east of) those Rolling Readers boxes ....and if he did that, then he could NOT decline the muzzle of the rifle low enough to hit a target down on Elm street.