I have looked at the low resolution flight radar data that is available. The only scenario that fits the information we currently have is flight control failure/malfunction, specifically the trim/horizontal stab. The go around profile and speeds are consistent with a normal go around - initial climb, then shallower climb to accelerate - the ground speed data is absolutely consistent on this. Then all of a sudden, at around a derived IAS of 220+ kts, a nose over and then an avg of 18500 fpm. Two pilots in a low authority gradient environment and neither one takes control? I can't see it. In my mind they were attempting to recover.
I see the comments regarding the parallels with the Kazan accident, but they already performed a safe go-around and they were already expecting to go around again. The disorientation is of course possible, but in my judgement so very unlikely.
As for fatigue. It may or may not be an issue in general terms. But this crew, from the limited info we have, they seemed to make sensible and logical decisions. They held because they had the endurance, weather was above minima with the worst condition forecast as a tempo, they clearly requested levels to keep them out of icing and they already made a successful go-around.
From what I can see, there is no evidence yet of a lack of SA or of poor judgement. No evidence of a stall and disorientation is so unlikely.