Flap extension is the most problematic time for a tail plane icing event. The conditions suggest at least the last part of the flight was in VMC, what conditions existed before that BO0M has commented on. Not that long ago, right over the top of Kunming, in winter doing one of their interminable arrivals, we descended through some some interesting AS, the kind you like to see in the ski season, but not in a plane. NAI went on before entry, and we had a camera running. We picked up 3/4" of ice in 5 seconds. In 28,000 hours of driving, have never seen ice pick up that fast. Point is, there are conditions that may seem benign, and result in severe ice accretion. If that happened, it would be discernible in the FDR data as a rapid change in the drag polar, and where there may be GPS data, then the change in CL can be quickly determined. Did that happen? Hard to say (easy to drink... Johnny Walker) The data will rule it in or out.
On the SEV ICE memo, there are a lot of natural cues that a plane is carrying ice, if the crew are interested in the proceedings, I am not sure that i have ever had any alert system that has not given erroneous data at some point... Would hope that peeps driving "plains" are never basing their continued happiness and well being on any single source being good.
As a profession we seem to "double cross the bridge" on stalls and our responses to that. Seems a tad untidy, and is hardly well received by the self loaders that pay our bills. A stall is not something that we should be fearful of, even in a T tail, like the B727, Cl600 or a Lear, or an ATR, stick pusher or not. Even a Trident getting parked vertically at Staines. Yes, if you abuse the plane for long enough it will get bitey, and the recovery from that, while quite possible needs a chunk of airspace between the pointy bit and the hard lumpy bits. The F-102 is about the most notable exception to that statement, it goes from tidy to untidy really really quickly. Still has to be abused, but, golly. The Staines, BAC 1-11, B-727 accidents wee all held in the condition by the crews for longer than the plane was prepared to put up with. Don't go out and stall the planes for the heck of it, the loads imposed on the tail alone are reason enough to avoid that, but the aircraft will stall, and will still respond correctly to controls for a period of time, relaxing back pressure will break a stall, up until the point that the stall has been abused so far that the bark turns to bites. The thrust line of the ATR should provide a nose down pitching moment on application, however, as that also blows the inner wing and flaps that can become a skosh non linear. The blowing effect assists in attaching flow over the flaps at larger angles which increases lift but also increases the Cm, the downwash increase induces flow at the tail that tends to increase the nose up moment from the tail, and that can become a bit of an issue, what controls is dependent on the tail volume, arm and the geometry of the stab relative to the engines...
FDR