Two quick items:
At Kegworth, the pressure vessel was ruptured, thus thrust went to zero (almost) instantly. Performing the shutdown checklist would have had no effect on thrust. Further, it was the secondary damage (fuel spilled) that caused the disaster.
Secondarily: Is there any case in which the fire handle should be pulled without a fire indication? It's been a long time since I had a FCOM or QRH handy - maybe someone can fill this blank for me.
I guess I just don't understand where you are coming from in the comment about thrust going to zero.
What pressure vessel was punctured.?
If it is in the fan or non-core pressure part of the engine then it only has a small effect on the rest of the engine, unless of course something else is wrong.
As for your last question. I asked my instructor the same and he replied that for a severe engine failure the assumption is that their could be a fuel leak and instead of waiting for a potential ignition source, pull the shutoff via the fire handles after you have cleaned up the aircraft.