Two questions that nag me.

[LR Trotter]

IMO, the slowing of the limo is very likely the result of a reaction by Mr Greer to the "Dark Compected Man" stepping into the street with a "hold it" fist in the air, just as "The Umbrella Man" was pumping the open umbrella up and down. Added to to the hard limo turn from Houston to Elm, the speed was already reduced and this occurred just as the limo should have been increasing speed.
As for the shooter of JD Tippet, I would think a description similar to the person known as Lee Harvey Oswald would somewhat match a number of individuals, especially considering the event being witnessed.