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?
To answer your first question:
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?
Not necessarily. One must consider a directionally controlled explosion.