Yes, a gambrel roof is the same as a barn roof. It is similar to a mansard roof, however the mansard roof has "hipped" corners and the gambrel roof does not.Hope that helps!
Yes
The English term 'gambrel roof' means a gable roof with two slopes on each side and the lower slope being steeper
Fire.
Neither is "better" in terms of usefulness or structural efficiency. Hip and gable are primarily responses to design needs.