Magnitos keep failing

[Forest#2]

But what about isolating the mag by running the kill wire through a horn button then to the engine block? Will that work?

Yes,

or disconnect the existing wires at the head light switch (isolated their ends by tapeing) and use ithe on/off headlight switch as you describe as a horn switch as a run kill test for awhile
Most likely replacing the ignition switch will fix your issue. I've seen it happen quite often especially if the tractor is parked outside and the key switch is exposed to rain. Loose rust forms down inside the switch causing intermittent voltage leak through to the mag and oil or contact cleaner is not the cure for such. The kill wire lead at the magneto is actually sensitive electronics inside the mag and any back feed positive voltage damages the magneto electronics.

 

[bertsmobile1]

You obviously have a switch problem
Believe it or not I have seen a similar case where it was ants in the switch
The problem is finding a good ground on mowers where everything is plastic .
If you mower has a carb solenoid and the solenoid works then you can ( for the purpose of testing only ) remove the kill wires , check that the solenoid shuts down the engine then mow like that for a few weeks to see if the problem goes away
Just be very careful because without the kill wires in place you can get off the mower with the blades running and new feet are a lot more expensive than new magnetos .