The speed will not be affected by incorrect orientation to North. It would just show the wrong direction. If it is not aligned North then you have the option of changing the direction in settings if it is too difficult to re-align the pole.
As for the difference in speed I find if I mount it too low compared to the surrounding vegetation or buildings I rely more on the highest value than the average because the average is pulled down due to the wind gradient and turbulence.
See my post here about differences in readings at different heights due to wind gradient and buildings:
cheers Ian 