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The way I read your descriptions it seems to conflict with your drawing.
At any rate, here are the facts as I see them, Option 1 on the EEE Dwg with the device on the rearward part of the light is 10 times more effective than Option 2 of the same drawing.
Sealing the gap would gain you such an insignificant result, you'd need a super computer, advanced wind tunnel, and a team of Formula 1 aerodynamics engineers to figure out the difference in Cd.
The whole idea is to get the air to make a clean break there at the back of the car so the air just flows smoothly instead of flapping back and forth in a wavy oscillation. So putting it forward would allow the air to try and reattach gaining you nothing or possibly hurting the Cd.
That said, the gain here would be very small even if you execute the shape perfectly due to it being such a small inefficiency that you're trying to correct.
The air in the "Boundary layer" within a few centimeters of the skin of the car is not the source of drag on your vehicle. It is the air out to 10 feet to the front, top and sides and 100's of feet behind being displaced by your car as you drive through it that is creating the drag. The drag comes from the energy required to move that huge volume of air, the more it moves, and the more of it that moves, the more energy it takes, and the less efficient it is. The most efficient shapes make the air move the least amount and it goes back to being calm within the shortest period of time.
There are papers out there that show how the rear edge radius of the vehicle affects the Cd. Very large and very small radius corners do well, there is a "Death Zone" radius of like 5 to 10 centimeters as I recall that really create a large drag. I'll try to find one the papers and post it later.
Hope that makes sense.
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