ste4en: You wrote "Looking at the 2*4 which burnt out it it is worse in two places, as if in the location of where the staples/clips that electricians use to hold the wire along the studs.". One of the causes of house fires is improperly applied cable staples, particularly a staple that was nailed down too tightly on its cable and pinches/cuts through the insulation to the copper wire. This can cause electrical arcing after further insulation degradation over time which can generate a lot of heat but not necessarily trip the breaker at all or not trip it quickly enough to prevent a fire. This is the point of AFCI (Arc Fault Circuit Interrupter) circuit breakers or receptacles, i.e. to detect the arcing quickly enough to prevent a fire. Although the latest NEC codes require them in bedrooms, etc in new home construction they unfortunately have not been perfected from what I hear (they are prone to nuisance tripping and are supposed to be checked monthly) and they are pretty expensive as well if you have a lot of circuits. I expect they will become more common over time, especially if they develop receptacle AFCIs like they have receptacle GFCIs -- but a problem is only an AFCI circuit breaker can protect the wire between the breaker and the first receptacle (and catch a staple problem there, for example).
One other comment: the heat from the fire would be expected to be conducted through the copper wire, so it is quite possible the receptacle/plug melted because of the fire and not because of a defect at the receptacle.