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Well, I suppose the unexpected simulation results are due to the fact that you're trying to use an op-amp as you would a comparator. Assuming you can, make the second op-amp a comparator (i.e. something which will accept a substantial differential input voltage, and which will provide a well-behaved logic output), and try it again.

The same goes for a real-life circuit: op-amps aren't designed to be comparators, and the mosfet wouldn't particularly want to see a large negative voltage on its gate for that matter.