I like your experimenting, and I'm following your development process with interest.
Here is an idea, it would add some complication, and may not be necessary, but I'll throw it out there. In order to ensure that the cord breaks, you could use a switch that is held in the on-state by the tension of the cord. Use the output of the Quark to trigger a latch which then remains in the on state indefinitely. The current path for the heater goes through the switch that is held on by the cord tension. Then the heater doesn't turn off until the cord breaks.
It does add a point of failure, if there isn't enough tension to reliable hold the switch on, then the cord will never break.
Here is an idea, it would add some complication, and may not be necessary, but I'll throw it out there. In order to ensure that the cord breaks, you could use a switch that is held in the on-state by the tension of the cord. Use the output of the Quark to trigger a latch which then remains in the on state indefinitely. The current path for the heater goes through the switch that is held on by the cord tension. Then the heater doesn't turn off until the cord breaks.
It does add a point of failure, if there isn't enough tension to reliable hold the switch on, then the cord will never break.