Why do you need such a high current mosfet ?
The voltage relative to the gate would be -12V.
I still think you should try connecting the "+" side of a 9V battery to the GND and "-" side to the gate.
You haven't had any luck with anything else . That should turn it off.
I wouldn't waste any more time with the TC4427, unless you are going to try running it off +12V, which
always worked for me.