Which side of the front steps did Baker ascend?
Where was Sarah Staunton standing, if Prayerperson is Oswald?
Which side of the front steps did Gloria Cavalry stop at, to make her exclamation? The side with people going up, with their backs to her, or the side where persons she knows, are standing FACING her, and are stationary?
If Sarah is standing behind BWF, during Wiegman and Darnell films, she is BLOCKNG the front door. Thus, Sarah would have to either move to the west towards Prayerperson or move east towards Pauline Sanders, to be out of the way of Baker/Truly entering the door.
IF Sarah leaves the landing entirely by entering into the front lobby, BEFORE Baker/Truly get to the door, then she would have likely been gone before Gloria Cavalry has reached the front steps. BWF remembers Sarah being "to his left" at the time when Cavalry has reached the steps and implies they BOTH heard Cavalry exclaiming. So this makes an early Sarah departure into the lobby, less probable.
If Oswald is PM. Sarah Staunton, moving west, almost into his space, and not remembering Oswald, = improbable. Moving towards PM, makes Sarah, even MORE an obstacle to the flow of persons coming up the steps= increasingly improbable. Not seeing Baker and Truly coming up this side of steps and them likewise mssing the 300 lb woman in their way= nearing zero probability.
Sarah moving east, away from the front door, towards Pauline Sanders,imo is the most probable hypothetical option if PM is Oswald. This is predicated on Sarah obscured from view behind BWF, having her LOS blocked, and thus Sarah missed seeing Baker,while Pauline, from her east corner, may have had LOS to Baker "approaching" across the 30 ft approx of sidewalk. When Cavalry reaches the lower step and probably stops to right side, where persons are standing facing her, then BWF and Staunton are looking in this direction. Sarah has now moved adjacent to Pauline, thus is between BWF and Pauiine. Both BWF and Sarah miss Baker ascending the left side, he now obscured into the "flow' of persons moving up.
Absent any other photographic evidence that more certainly identifies Sarah Stauntons location on the landing during the time of Wiegman and Darnell film capturing the image of PM, or some other evidence placing Sarah Staunton in the front lobby before Truly/Baker enter, then the above hypothetical logical deductive reasons given by Zeon Mason, must remain speculative