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This is pretty long-winded, so apologies if I'm going over old ground or stating the bleeding obvious :)
When you say that the engine dies instantly, do you mean that it dies the moment the ignition key is released or does it hang on a bit longer? If it dies as soon as the starter motor stops running then you might want to check that the ignition system's ballast resistor is ok. When starting the engine the ballast resistor is by-passed and the ignition coil is provided with full battery voltage. The moment the ignition key is released the ignition system current for the coil is switched through to the ballast resistor. If the resistor is dead, the coil gets no current once the ignition key is released after starting and the engine dies.
If the engine hangs on for a bit, then staggers and dies, the problem isn't the ballast resistor.
In what way is the coolant temperature sensor wonky? The sensor's electrical resistance is designed to vary with the coolant temperature. Eg: At 10 deg C the resistance should be between 3000 and 4500 ohms. At 40 deg C resistance should be between 1000 and 1400 ohms. An "open-circuited" (ie infinite resistance) sensor would certainly make the engine run rich, but I doubt whether it would run so rich as to die completely. If the sensor has short-circuited internally it would run the engine very lean. To test the resistance of the sensor accurately you need to know the temperature of the coolant (or the air temp if the sensor has been removed from the engine) and make sure that the two electrical contacts on the sensor are very clean.
If the sensor is definitely at fault, then ignore the rest of my rambling from here on :)
Next suspect would be the cold start injector, an "extra" fuel injector fitted to the top of the intake manifold near the throttle butterfly. It performs the same function as a choke on a carby, and dumps fuel into the engine when its cold. However! It should only function when the coolant temp is low and only while the starter motor is running. There is an electrical contact on the starter for the cold start injector which provides it with current only while the starter is engaged. The current then travels to a thermal timer which is a temperature controlled device, a bit like the coolant temp sensor, which only allows current to procede on to the cold start injector if the coolant temp is sufficiently low to warrant it. It also has a timing function which limits the time the cold start injector runs for. The cold start injector itself might check out perfectly ok electrically (4.2 ohms resistance at 20 deg C) however it may have failed mechanically. The electrical part of the system might be perfectly ok, but the injector could be stuck open. Best way to check is to remove it from the manifold (easy; couple of screws), unplug the electrical connection, then turn the ignition on and off a half dozen times (without starting the engine) to run the fuel pump and build up fuel pressure in the system. If the cold start injector has a mechanical fault it will leak like a ba*tard and spray fuel. That is not good. They're not repairable so replacement will be required.
Another item which might be at fault is the manifold air pressure (MAP) sensor. Electrically it should have the following values: Resistance in primary winding (terminals 7 and 15 on its socket) approx 90 ohms, resistance in secondary winding (terminals 8 and 10) approx 350 ohms. Even if it is ok electrically, it can fail mechanically. It has a metal diaphragm inside which can split. The guts of the MAP sensor should be absoulutely airtight and a split diaphragm will cause an air leak, messing up the entire function of the sensor. Quick check is to remove the vacuum hose which is connected to it from the intake manifold end and suck on it. It should hold a vaccuum without any leakage.
Other than those areas, it is vaguely possible that the electronic control unit (ECU, processor, computer, etc) has a problem and is stuck in a mode which is telling the fuel injectors to stay open permanently. Very unlikely, but possible.
D-jet is great while its working, but can be a pain in the rear when it acts up.
Good luck.
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