As far as battery size, other car companies can't do it because they want to share as many components between their ICE cars and electric cars as possible, which means no skateboard platform. If you reduce the model S down to 3 series proportions, the area between the wheels where the battery lives only reduces by about 10-15% (the 3 series wheelbase is only 6 inches less than the model S). So with today's battery technology instead of an 85kWh battery you could only have a 70-75kWh battery, which would obviously be fine.
There are some companies with EV offerings that appear limited by their ICE siblings, but not all of them. And Tesla is not alone in its use of the skateboard. Here is the i3's layout, as well as the i8. Both use the skateboard.
But the i3 is only able to turn that into a 22 kWh battery, while the i8 is only 7.2 kWh.
BMW i3 Electric Car: ReX Range Extender Not For Daily Use?
It appears that Tesla is able to do more with battery volume.
But I can't quite agree with your math. The skateboard has to fit in between the car edges and wheels. in the back it sits just in front of the wheels, while in the front it extends to in between the wheels. And in the back, the motor also needs to find space.
And the battery can't extend all the way to the edge of the width of the car, given that there are other surrounding parts that can't so easily be squeezed (e.g, the metal around the battery, as well as the doors and what is inside them). So squeezing the total width from 77" to 70" is likely more than a 10% decrease in battery size. Maybe15%? But the problem is that we are not in a one-dimensional world. One also must shrink the Model S from 196" long to 182" long to be 3-series-sized. So 85 kWh becomes 72 kWh (85 x .85)
http://en.wikipedia.org/wiki/BMW_3_Series_(F30)
That's another 7%, which, again, includes some "tough to shrink" parts like bumpers and lights and the like. So call that another 10%. 72 then becomes 65.
Is there a need to further cut down on the vertical dimension? I'll assume not.
All that said, I agree that a 65 kWh battery will potentially do the job, and that's with no additional battery tech improvements. It just causes me to wonder why others aren't doing this. I mean, if BWM would increase its i3 battery from 22 kWh to even 35 kWh, the car would be SO much more compelling.