Stair skirt board protruding from wall by ~1/4" - can I use a belt sander to sand it down?
This isn't too tough. Make your life easier by finding a plinth block that coordinates with your rail system or other nearby trim components:



Use whatever tools you have on hand to trim the skirt back about 3/4" from the wall corner. Fit the plinth block over the end of the skirt to act as a cap. This way you don't need a perfect cut on the skirt, and you don't need a miter return into the wall. 

A technique I've used for this is to rabbet the back corner of the plinth block using a table saw. Run from the bottom upward, and stop short of the top of the block. This leaves a curved notch that can fit the top profile of the skirt rather nicely.