I think your magic trick will be to insert a 18V zener diode. They work in a way, that current will flow in one direction, and no current the other way (like a normal diode works) but when the reverse voltage exceed the zener level (eg. 18V) can current flow in both directions.
In your case, where a protection diode potentially make current flow from a relay power back toward the FET, throu a protection diode, and back to a mains somewhere, can a zener allow the spike protection, but stop the reverse current a normal voltages (12-14v).
It's a common issue when the design controls the electornics on both the positive and negative side. Even I thougt that I was carefull when I designed the complete loom for my TVR, did I end up with the nice feature that turning on the long beam, with the key out... made the ECU power up..... a couple of hours and a few diodes later, did it work as expected :-)
Injector flyback, that is powered from the injector +12V should normally not have power without the ECU is on power, Either by using the same relay for both injector + ignition + ecu power, or by powering the injector and ignition with the fuel pump relay
Br Mads
