Yes, kill switches do fail. and short circuiting them is the cheapest fix. They tend to get dirt and corrosion inside, and often the engine will cut out going over a bump or similar, or wont start until you fiddle/tap the kill switch. As regards the throttle closing cable, just replace the twistgrip tube with the correct item with 2 cable slots in it (e bay or d.s.s., around a tenner) and then fit the second cable.