4. Also - unless there was some other flammable material in the top of the fuselage amidships - where the fire flared up considerably about 5 minutes into the 2nd video - it would appear that composite fuselages flammability might be a concern.
Excellent post overall.
Not sure what you mean but I'm pretty sure that the trip-7 does not have a composite fuse. Composites are said to be used in the tails (h/v) and the floor beams.
Boeing: The Boeing 777 Family: Preferred by Passengers and Airlines Around the World
Even the upcoming 777X is going to have a metal fuselage:
Boeing Defends 777X Aluminum Hull as Plastic A350 Flies - Bloomberg