As in the usual way in these matters, the more we learn the more questions arise .....
ACARS shows data transmissions continued for 3 minutes from the first "window overheat" before all power was lost (to ACARS)......
90 degree turn is a standard procedure to leave an airway if you wish to make an emergency descent - it gets you out of the way and avoids you flying into someone below you - tight 360 degree turn at high bank angle is also a standard procedure to lose height rapidly in an emergency descent.......
Smoke/fumes/fire would all instigate an emergency descent........
So, it appears possible that an emergency descent was commenced due to smoke/fire in the cockpit/toilet/avionics area ..... BIG question that doesn't compute .... why no radio calls
The avionics bay was powered - to ACARS - for 3 minutes, the aircraft was flyable and under control as it did those two manoeuvres so the pilot (s) were conscious and operating effectively, but no emergency radio call, no IFF/SSR emergency squawk, nothing ......
3 minutes is a LONG time in an emergency situation to NOT instigate a "mayday" call or to NOT press the 7700 squawk button on the IFF/SSR box ... even if only one pilot was working, training and instinct would suggest those calls would be made .... but they were not .... most odd ....
Rest in Peace all those lost ... we will learn what happened eventually.