Why is the Oort cloud presumed to be spherical? Most descriptions of the Oort cloud depict it as a mostly spherical distribution of planetesimals, with occasional allowance for an inner component that is more donut-shaped. This is slightly at odds with the fact that most protoplanetary clouds and their derivative objects - planets, asteroids, comets and dust - will collapse to a fairly well-defined plane relatively early in a stellar system's evolution.

What evidence is used to postulate this? Does it come from numerical simulations of the solar system? Or does it help account for observed orbital inclinations of real comets?
The latter.  Long-period comets appear to come from random directions.