 Regarding the site survey, whichever day it happened, Crabtree says that a Secret Service advance team normally walks the protectee's route in and out of the site and then stands on the stage, much like you said, looks out 360 degrees to determine vulnerabilities, keeping in mind handgun and rifle effective lethal ranges to make sure those vulnerabilities are posted and covered. Somehow they failed to properly account for and protect a rooftop, a prime sniper position, some 400 yards away.