Shifting: the starter and the shifter arm have a safety interlock. If the starter button is sticking, that is (hopefully) the problem. If the starter button gets fixed and the problem continues, try removing the battery from the circuit and use the kickstarter. If that fixes the problem, replace the solenoid? or whatever it is that keeps the starter from engaging if the xmsn is in gear. If that doesn´t fix the problem, you have xmsn issues serious enough for a rebuild.
Starter switch screw: use an eighth-inch drill bit and drill out the center, then tap in a bolt-out (looks kinda like a cut nail) and unscrew the messed-up screw. You will of course have to replace the screw, so try and keep one with threads so you can get a perfect match at the hrdwr store.