Again, I very much appreciate your thoughts on this as I am slow to commit to my math (I often make stupid errors). I had presumed that the air would not provide much of a buffer as the solar wind has a measurable effect on position of the van allen belts and yet has negligible effect on the earths orbit over billions of years.
If the time scale on the decay of the orbit is in thousands of years, the pressure due to the shock wave would have to, at the least, be millions of times greator then the current solar wind.
From your numbers,if the force is given by DSv^2 (which seems very reasonable to me), then the pressure at the surface is in the ball park of Dv^2. Using a density of 1.4x10^-5 Kg/m^3 (roughly your lower estimate)I get a pressure on the order of 100 thousand tons per square inch (I like british units I can visualize them better than metric units).
If I didn't make a mistake in a decimal positon, that is an extremely large number when compared with ordinary surface phenomen including gravity itself. A mile of solid steel provides a pressure of only about 5 tons sq inch so it would be equivelent to a static loading of about 20 miles of iron.
It would clearly blow the atmosphere away and in fact would create major changes in the shape of the earth. Those changes would then lead to increases in the effective cross section effectively generating explosive disruption.
I could be wrong, what do you think?