I thought this was ultimately about whether the limo stopped West of the triple overpass? How it's about whether Kellerman could fire back. For the record, I will state that the motorcade's first line of defense is its mobility, not it's firepower. That's apparent in the Reagan assassination attempt, for reference.
BTW: Greer, Kellerman, and the guys in the Queen Mary were the protective detail.
Well, if You believe the JFK Limo boarded the Stemmons Fwy On-Ramp in front of the Lead Car, where do you think SA Greer thought he was going/headed? He was unfamiliar with the Dallas area. The JFK Limo could have easily been flushed out of Dealey Plaza and headed straight into a well planned ambush. If you believe the Limo boarded/sped down the On-Ramp ahead of the Lead Car & the Queen Mary, that protective detail Behind the POTUS is useless. The JFK Limo had to Stop, Regroup, & Reaccess the situation.