Keep in mind that people will need these stations far less frequently than they need a gas station. All of your day-to-day charging miles come from your garage. Only on the occasional long trip will you need a SuperCharger.
That being said, there are other ways to address the problem. The biggest one is that non-Tesla fast chargers are also being built and Tesla will have an adapter for those. Also, I believe I heard that they may allow you to see on the smartphone app if the stalls at a specific station are in use, but don't quote me on that. It seems like a pretty simple feature to implement though.
Also, I don't think assuming that the number will be based on average rather than peak load is correct. If that were the case there would probably be 1% of the stations that there are now. Same reasoning applies to why there are so many pumps at a gas station.
That argument is irrelevant. The whole point of these SuperChargers is that they would be located along interstates to make long distance driving possible.
Obviously if you never go more than 20 miles from home, you will probably have no need of a SuperCharger.
The smartphone idea is a good one, but useful only if you have the flexibility to choose. If there's only one station within range, you're stuck with it.
Tesla has pushed hard early on to ramp up the number of charging stations as a sales incentive. As the construction rate trails off, and the number of cars increases, that's when the problems will start to crop up.