Pete, seems like you've covered everything from a servicing perspective, just about the only thing you havent done id=s remove the head and clean off the carbon, which is overkill, unless you think it really needs it.
The start stop arrangement is already set up for safety purposes, the only other way to kill it if it fails is to pull off the plug lead, preferably with some insulated pliers.
The setup is quite simple, under the throttle plate assembly there is a switch with a wire on it that goes to the underside of thecoil(on the carby side), When you fully close the throttle the plate moves around and makes contact with the switch, hence shorting the low voltage side of the coil, exactly as a 2 stroke. Pretty easy really. The switch is more easily visible with the throttle assembly removed (the top 3 head bots and the front carby bolt hold it on). You're learning, so pull it off and have a look around, its best done with the fuel tank and air filter assembly off so you can disconnect the governor spring(s) and trace the kill wire to the coil easier.
I hijacked this picture from someone elses post
[Linked Image]

The bit where the arrow point to is where the contact point for the kill switch is; if you move the throttle plate you'll see it. The plastic bit in the highlighted box is the kill switch assemmly mount, it just pushes through a hole in the throttle plate assembly.
Cheers and good luck.