The ghost in your case resides in the yellow / yellow-black wires
From the circuit diagram you see the magneto gets killed via the seat switch & the parking brake switch
The path is ground > green wire >brake switch >yellow / black wire> Seat switch> yellow wire > magneto
When the brake is off the green & yellow / black wires are connected together so the seat switch should show ground on the yellow / black wire side
If the seat witch is working and you are in the seat there should be no connection between the yellow & the yellow / black wires
If you are not in the seat then the yellow & yellow / black wires get connected & the magneto is grounded .
Take the seat switch out and check that is is open circuit plunger down & closed plunger up
Start the engine and release the brake or jump the green to the yellow black at the plug ( it will not crank if the plug is removed first ) then manually hold the seat switch plunger down , then let it off every time you let it off the engine should shut down
If the switch checks out OK then it will be the wiring ( yellow wires ) from the seat switch to the magneto
On your machine you check this by unplugging the seat switch & starting the engine then jump the plug
Every time you jump the plug the engine should stop if it stops with the plug open then there is a short to ground between the seat switch & the magneto.
Seat switches on ZTR's take a pounding and do give more troubles than on tractors
There are two cut out circuits that run in parallel with each other
PTO + SEAT + MAGNETO
BRAKE + SEAT + MAGNETO
PTO & Brake are not in a circuit themselves but both provide a ground to the seat switch