This reminds me of the Air Canada DC-9 about 30 years ago. A fire started in the aft lav, and in the 20 or so minutes it took to get on the ground and evacuated, 22 people were dead. From the burning interior materials, all had fatal levels of cyanide in their blood.
Blood tests of those on the flight would be worthwhile.
If the IFE, In Fright Entertainment, box under 34ABC were shorted and cooking, it's possible that killed the PA to several sections of the cabin. IFE is an afterthought in airliner design, and is often retrofitted with the latest and greatest to match the competition. Since it is classed as nonessential equipment, it doesn't get the same level of safety analysis as the avionics downstairs and in the cockpit. Not to mention SR111.
The PA is at least at the safety level of Essential. It's even live when the plane is down to running on batteries. It's possible in this case there wasn't enough isolation between the PA and the IFE.
When I first got peripherally involved with IFE a dozen years ago, I was shocked to learn they pipe 115vac to every seat group, and those seat boxes get very hot. It never should have taken that level of waste heat to deliver 100 milliwatts of audio to 3 seats. The stuff on BA should be more efficient than that, but apparently not.
GB