I am a little bit surprised that nobody has mentioned the speed trim system in this conversation.
If indeed the horizon-stab ended up positioned to fly the nose of the aircraft down, then the STS is one of several ways that it can get there.
The horizon-stab could get there through some sort of failure.
It could get there by being manually motored there by the PF.
It could get there by being motored there by the STS.
What conditions would result in the STS motoring the stabiliser 'aircraft nose down' ?
1/ The A/P must not be engaged
2/ High thrust settings and light weights
3/ Airspeed decaying below target speed
So when we see 6000fpm climb, I wonder if an increasing headwind has resulted in higher attitudes being flown and when the head wind stabilises that attitude is too high, speed decays, STS try's to compensate by trimming forward.
If this did occur then another hole has lined up as well because elevator should be able to control the attitude even if this has happened.
Where you would be in trouble though is if you let the nose get too low in conjunction with the ' nose down stabiliser' and then tried to recover solely with elevator.
My understanding is that elevator can override the stab position but if you have an aircraft pointing downhill and are trying to bring it out with the (relatively) small elevators, you'd want either lots of height or not too much nose down attitude.
That's just my thoughts, I'm not suggesting it applies to this flight ( or even that my assessment of elevator capability is accurate) but I am surprised that the STS hasn't been discussed yet.
Thoughts?