No problem reversing inserts if you think it'll help, but remember that you can't reverse the holder because the closed end has to be to the front. Otherwise the pad may eject when you apply the brakes.

FYI- perfectly adjusted pads would tend to wear most in the rear, because of the torque applied by the forward moving rim. That's why Kool Stop moved the mounting bolt forward (or lengthened the back depending on how you look at it). If the front of your pads are wearing more than the back, you probably have too much toe-in, and should remount the shoes more parallel to the rim.