With the car moving away at a slight angle after making the turn but before reaching the underpass, the shots would have been reasonably easy from the depository window. They were all well within range for any rifle, and would not have required any elevation adjustments or more than minor windage adjustments.
In contrast, shooting from the knoll, sewer grate or other alternate sites would be far more difficult. From there the car would move across the shooter's field of vision, requiring dramatic windage adjustment and the ability to fire a rifle like a shotgun or time a single shot precisely. I think folks tend to overestimate the challenge posed by distance and underestimate the challenged posed by a target moving laterally across your field of fire.
There are some conspiracy theories associated with this killing that could be made into outstanding movies. Espionage, cover-ups, a secret assassin, and plots withing plots. But this isn't one of them. The alternate shooter locations are the *least* interesting and among the most absurd of the conspiracy theories. Even if LHO was eating his lunch at the time, the shooter still would have been in that position or one very similar. It's a very good spot.
