I MIGHT BE WRONG but think what you assume is "the street visible between the legs of [some of] the people who are on the Island" is actually the Island, and the reason we can see "it" is because the car isn't parked on the Island, it's parked on the street right next to the Island.
I am currently looking at the Wiegman Film from Gary Mack's "Unsolved History JFK, Death in Dealey Plaza" DVD. You previously mentioned the older guy wearing a Fedora standing atop the Island. To his (R) is a lady with a purse hanging from her (L) arm. There is open space between this Purse Lady and the Fedora Man. Through this open space I can clearly see all the way back to that car parked in front of the Huge Gates on the other side of the Elm St Ext. I can see the Top, rear window, (L) rear fender, (L) rear tail light, and the upper portion of the trunk lid of this car. My seeing this car would be blocked if a car was parked directly behind the Fedora Man and the Purse Lady alongside the island. Maybe your error is due to the inferior copy of the Wiegman Film that you are basing your conclusions on? The discovery of this car moving down the Elm St Ext while the JFK Limo was still on Elm St is a major find. The "Coupla Guys" walking away from that car on the Couch/Darnell Films is extremely suspicious. I believe they are attached to that car.