Oswald in the doorway: the blog of the Oswald Innocence Campaign, by Ralph Cinque

NO, you blithering idiot. Brehm WAS always in front of Babushka Lady, meaning that she stood back, and he stood forward. He was close to the curb; she was distant to the curb. In this shot here, she is shooting diagonally at the limo as it approached her.

By the time, the limo got close to her, Brehm was clearly in her way.

BL had to wait until the limo got past Brehm before she could shoot again. So, she had turned, and she was poised and ready to shoot as soon as the Kennedys came into view.  She was waiting.

BL was trying to get as many photos as she could. So, she shot before the limo reached her, then she waited through the time that Brehm was blocking her view, and then she resumed shooting after the Kennedys re-emerged from being obscured by Brehm. 

No one who was going to take ONE PICTURE, as Mary Moorman was, and who was ready from the moment he made the turn at the top of the hill, would wait to take her ONE PICTURE until after he had passed her. No one would do that. Nobody.  

Now, as to why BL chose to stand back behind Brehm, I can only speculate. I suspect BL was in on it, that she knew it was a Kill Zone, and she didn't want to get too close to it. Better that Brehm get hit by a stray bullet than her. 

And no! Mary Moorman would not have been blocked from shooting to the east in this picture. You are being confused by the two-dimensional rendering of a three-dimensional space. 

Look at it in Zapruder.

The Kennedys hadn't reached her yet, but she certainly could have shot them. There was nothing obstructing her view. Then, or very shortly after that is when she took her picture. She certainly did not let them pass her, and she has never said that she did nor demonstrated that she did. 

So, to answer your question, no one who was taking one picture and one picture only would wait until her subjects had passed her to take their picture. But, Babushka Lady was taking many pictures. Many have assumed that she had a movie camera, but she didn't. It was a Sears Tower still camera or one like it. But still, she took a lot of pictures, some before they reached her, and then some after they passed her. She could not photograph them when they were directly in front of her because of Brehm.