So tell this Forum your own explanation for ALL of this. My "challenge" stands.
I don't have an explanation because there isn't enough information to form one. I can speculate about a possible scenario without stating it as if it were fact which is what you do.
A car started down the Elm St. extension, only to be stopped as the motorcade reached Houston and Elm. Once the shots were fired, there was resulting chaos. Traffic came to a standstill and the occupant(s) realizing they weren't going to be able to exit any time soon, simply parked the car and left on foot to wherever they were intending to go. They came back 3 hours later after things had calmed down.
There's as much evidence for my scenario as there is for yours and while yours sounds more interesting, mine is mundane and far more plausible. It doesn't require me to assume there was a sniper team inside the TSBD for which there is no evidence.