In the (my) usual manner of judging likelyhood by what seems to have happened this has all the hallmarks of the classic B737 dual coupled approach to manual g/a pitch trim gotcha. It's just such a good fit it's hard to imagine another scenario.
Walks, talks and sounds like a duck I'm afraid.
So my guess it's a duck but time will tell.
But why on earth hang around for 2 hours in the hold when there seems little chance of improvement and there are viable diversions not far away?
Surely 2 hrs in the hold tells you something....?