[Alan Anderson]

The appropriate answer to #2 is to mechanically support the arm so that gravity is not a large influence on its motion. Either a counterweight or some sort of spring arrangement would work. There is no easy way to deal with it in software, especially with a window motor.

For #3, setting the P constant too low will result in the motor being given so little power that it will stop long before it gets to the set point. The I constant can compensate for this, but it will take some time, and is likely to overshoot badly. What you generally want to do is increase P until you get oscillation, then back it down until you get reliable motion with a reasonably small steady-state error. Then increase I until that error is resolved with a minimum of time and overshoot.