One thing I would note,
Where are both sides of the Horizontal Stab? Its certainly not on the shots with the vertical stab.
Would certainly explain the speed loss and height gain if the stab has gone in the cruise plus directional issues if a major part of the structure has failed, particularly stab pointing up into the airflow... aircraft would otherwise want to constantly nose down if the stab had gone completely.
I would be very interested in any recent maintainance/repairs of this area (rear bulkhead) as well as any historic damage.
Jet suffered tailstrike damage in 2001 for which it received repairs. If I were investigating, I would be spending a fair old amount of time looking for any cyclic metal fatigue and crack propagation around the rear bulkhead area, any pre-crash impact damage with the screwjacks...
Something similar happened with aft bulkhead damage caused by a vehicle on the Bagram 747 freighter crash a few years ago, pitch up, irrecoverable and uncontrollable stall.
this pic is pretty telling, especially the direction the fuselage material is bent (outwards)
Also, an aircraft with a freely floating stab will incur opposite control motion (if the elevators are controllable still) to the input. Interesting Gen regarding stab damage on this NTSB report: Aircraft Accident Report, AAR-15-01
RIP.