It’s not parallel, it’s hitting the same point the south bleacher.
The quality of image is very poor here.
Doing your scenario it does not hit the rail.
In this post also concluded the same think from James Copenhaver footage.
If the line of sight from James Copenhaver footage is almost aligned with Trump ear and the Vent that means that a fire from the vent would pass at the height of the camera eyepiece. Therefore, if the bullet hit the rail it has come from an upper position.
I also explained that here to @roger-knight, where I did a model correcting Google Earth height using James Copenhaver footage.
In his opinion I cannot use the footage for the model, but I think that it still proves it did not come from the vent, but from an upper position.