You're not mentioning the rate of the gas outflow from the comet's nucleous. That, along with velocity(of the comet) tends to give a 'meandering' comet tail effect.

Just one of the many reffences out there:
http://www.sprl.umich.edu/SPRL/research/atmos_of_planets.html

[Studies of the neutral comet atmosphere, or coma, have enabled Laboratory scientists to understand various aspects of the abundances of gases, their dynamics, photochemistry and production rates from the nucleus. New simulation methods developed at the Laboratory are now being applied to the interaction of the cometary ionosphere with the solar wind. With the help of these methods scientists expect to model the entire interaction region from the surface of the nucleus which is a few kilometers across to the whole cometary ion tail which is tens of millions of kilometers in length.]