I'm not a Dash 8, or icing expert. But I'm wondering...
The posted photo of the Q400 tail shows the fairing, which projects forward of the intersection of the H and V stab leading edges, and it is not apparently deiced.
25 years ago, I had a terrifying experience flying a quite new Cessna 303, whose H and V stab leading edges form a cross at the intersections of their leading edges, which on that particular aircraft was not deiced (though the aircraft was known icing certified). We were flying in icing conditions, and accumulating ice, which the boots were handling fine. After a while, the aircraft seemed tempted to swap ends in the pitch and or yaw axis (roll was perfect). As I slowed down, it got very much worse very fast. I sped up, it got better. After finding warmer air, the ice melted off, and we lived. The aircraft had no ice upon landing, and flew perfectly. The AD on the c303, which addressed this design shortcoming of the aircraft, made reference to complete loss of control (in pitch I'm betting), and resulting in flight break up, with fatal results.
Could something similar be happening here? I've got to think that this has been addressed during icing certification of the Q400, but it was overlooked during the initial icing certification of the C303, and only found after accidents....
My contribution to the speculation for the evening....
Pilot DAR