I measured it in Gimp. Adams and Styles would have travelled about 93% of the distance that Oswald would have. So, no, not considerably less.
When Mrs Garner said that when the girls went to the stairs she followed them from the office to the storage space, she also said that they had already started heading down. She never saw them enter the stairwell.
I measured it in Gimp. Adams and Styles would have travelled about 93% of the distance that Oswald would have. So, no, not considerably less. I think you are overlooking the door between the office and the storage space right behind where the girls were watching through the window. In the drawing you've got them going the long way around.
But regardless, if the girls and the shooter both left their locations directly after the shots, even by your measurements, the girls would still arrive at the stairs before the shooter. And that's not taking in account that Brennan said the shooter did not leave instantly.
When Mrs Garner said that when the girls went to the stairs she followed them from the office to the storage space, she also said that they had already started heading down. She never saw them enter the stairwell. If she said "they had already started heading down" she knew they were already on the stairs, regardless if she actually saw them or not, right?