Steve,
I noticed something today while flying a different aircraft with IF that I had been having troubles which were similar to the Mustang - the LotusSim L-39 (military jet trainer). After thinking about the issue with the fuel cut-off on the Mustang I remembered that the L-39 has a similar device in the cockpit.
When I loaded FSX without IF and then loaded my default C172 cold and dark scenario and then loaded the L-39 the fuel cut-off was disabled and I was able to startup the L-39 according to flight procedures(see below).
When I loaded the IF scenario and FSX the fuel cut-off was visibly enabled preventing me from starting the engine (below).
I would guess that something in the IF .pln file (or any others used to start the flight) are affecting the fuel cut-off device. This is similar to the Mustang except that you can't see it so obviously with that aircraft.
My IF cold and dark setting had only the parking brake, a/c fuelled and knee-board checked.
Regards,
Brent