If there was an explosion prior, wouldn't the debris field be much different? Shouldn't there be a massive amount of debris 500+ feet or more before the impact site? Especially if it was powerful enough to rip an engine out of a car?
Look at these two crashes done by mythbusters:
50MPH vs 100MPH
Notice the level of disintegration at 100MPH. In the mythbusters crash the engine was not running and there was no fuel tank. In Hasting's 100MPH crash there was likely ~16 gallons of 92 octane fuel and a rather large engine spinning at 5000+ RPM.
Could it be possible for the engine to be ejected at the time the car disintegrated at the tree? Combined with a gyroscopic effect and a still spinning drivetrain is it possible that it could have rolled itself backwards 200 feet after the crash?
I only bring this up because using an explosive probably is a dumb thing to do if you are trying to fake an accident.
It would make much more sense to me to take over the electronics with a payload that runs in RAM only. Once power is disconnected by the crash all traces of it are forever lost. If this payload was able to prevent the data logger from recording at the time of execution there would be zero evidence of foul play in the black box (if it happened to survive the ensuing fire as well)