If a larger screw doesn't work dropping the dash is about your only option....it really isn't that bad, just the screws on top, the two 13mm??? bolts underneath, and the steering column. You may not even have to take the column off of the firewall to pull the dash back far enough.
I ran a thread die over all the seat bolts and seat belt bolts along with a bunch of PB blaster and I didn't break a single bit on my truck. The die made a big difference in how they came out.
Try reading this thread: http://www.thetruckstop.us/forum/showthread.php?t=13891&highlight=door
That is the basic instructions for removing the door panel(s) other than the instructions there I am not sure what could be holding you up.
You can check the motor and arm from under the dash. Have someone run the controls and watch the motor to make sure that it is working properly and check to see that the arm is still attached properly and not binding. If it is actually the door itself, you have to take the dash down and split...
Make sure to drop the rear suspension out ever once and a while to look it over and grease it up!! Espically the main pivot, don't let the moisture get in there, I have seen several of them lock up and snap off.
That sled should serve you pretty well, the Polaris 500s are some of the best...