I keep coming back to physics first principles:
I imagine having solar on cars as a way to increase general energy flexibility in transportation, and as an offset to energy use and land use. But, in the above exact calculation, that person with only a 15 mile commute to work would actually no longer need any additional energy for their commute during summer. Whenever they plugged in at home, it would unnecessarily use shore power (home solar, utility power) to give insurance that they will have more SOC after work or to make up for extra trips; they certainly don't need it to get to work or get home in regular commute. During winter, the driver would have to plug in more often.
When do I think the costs will drop to reasonable? After all the above has been completed and more time passes during which more R&D continues, something that could be done by selling less cost efficient systems to the types of people who used to buy Roadster, Model S and Model X. So, I'd say about another 3 - 15 years from above, so about 6 - 30 years from today (plus pause time) before this is cost reasonable. That's within our lifetimes, if pause time is assumed to be low (and, at the extreme optimism end of the scale, is before Trump is out of office). Put another way, if no pauses are taken, by the time we have a colony on Mars.
Good calculations of the theoretical maximum, but everything would have to be optimal for you do see anywhere near these numbers and there are some other factors. If the entire surface of the car was used, there would be a good portion of the car surface that was not generating energy at any point in the day. The only portion of the car that would get sunlight most of the day would be the top, one side of the car would be mostly in shade for half the day. Additionally the car would have to be clean all the time and this optimum would only be achievable on the sunniest days.
Ultimately it would probably be more efficient and practical for the employer to put a solar roof over the parking area at work and offer DC chargers for EVs. The charging efficiency will be better when the car is charged directly from DC coming off the solar panels, but the roof being flat instead of curved like a car body will have a more efficient angle with the sun. The area directly over each parking spot can probably generate more power than a car in that spot could ever generate.
With a stationary installation, there is less need to miniaturize equipment, the cars don't have extra weight from the solar equipment, and any extra energy generated can be stored for later use or put back on the grid. On the weekend when there are few if any cars charging, the power generated could be stored in a stationary battery array for charging up cars during the week.
A small solar panel on a car does have some use. It can be used to offset vampire loss sitting in the parking lot during the day, or if parked at the airport while you're on a trip, to ensure your battery isn't drained when you get back. Another use could be to run the A/C to keep the car a more comfortable temperature during the summer. This would be useful for someone who is running errands with their dog or just someone who wants to get into a nice cool car on a summer's afternoon after a long day at work.
The energy needed to compensate for vampire losses and running the A/C especially for only a part of an hour is a much easier problem to solve than actually getting enough energy to drive the car. You came up with 368Wh/Mi as the average for Tesla owners. I note that's an early number and my number is around 310 Wh.Mi with my 90D, but I haven't been through a winter with it yet either.
Assume 333 Wh/Mi for convenience, that's 1 KWh for every three miles driven. The average US household uses about 11 KWH a day. That's enough energy to drive only 33 miles. The power needed to move a car is pretty large compared to other uses for electricity. Running systems while the car is sitting idle is not that difficult, actually getting much power to drive the car from solar built into the car is probably never going to be as practical as plugging the car into a stationary solar installation.
If solar powered parking lots become common, it has the added advantage that finding a shady spot to keep your car cooler on a summer's day will be a lot easier and charging up your car whenever you park will be possible.
Yes I think development is possible, like stacking 10 thin layers of solar cells on top of each other to get out more energy per area?
Only the top layer will work. The components that actually absorb the sunlight are opaque, so the lower layers would be shaded by the top layer.