Two points which have been made before and possible stand repeating.
Tail stalls happen because the disturbed air from the ice build up cause a low pressure pocket of disturbed air on the lower surface of the horizontal stabiliser. When this bubble reaches the elevator it pulls it forward.
The Q400 has an all powered tail. The elevators are not moveable by aerodynamic forces at all. only by the hydraulic rams. All you have at the front end is a spring bias to feel the forces.
Secondly, the Q400 has a fair amount more power available than most if not all commercial turboprops. Firewalling or even selecting the detent when MCR would have been indicating on the ED would give a massive amount of lift, up pitching moment and also left yaw. Performing go-around is the sim when trimmed to 20 kts higher is still a fight, let alone trimmed to the stall...... Countering a lot of left yaw with too much right rudder, while just at the stall, with loads of power and a pitch up force sounds just like a classic spin entry manoever to me.....
However, the actual sequence of control movements will come out in the report and before drawing conclusions on this specific event, we should wait for that.
Personally, I'm more than happy to go fly a Q400 anyday.