I replace stems without removing the wheel or tire. I squeeze the tire by the stem with a large C-clamp until the tire pops off the bead shoulder on one side, reach in and remove the stem base after cutting off the stem top flush with the rim.
I have yet to balance any of my scooter wheels and have not noticed any vibration, shimmies or weird wear as a result, despite riding at 70mph regularly. Have I just been lucky? Has anyone out there had problems with an unbalanced scooter wheel? 17" wheels on full size motorcycles definitely need balancing but scooter wheels, not in my experience.