I have expressed myself somewhat vaguely, my apologies. I did not mean to infer that Q400 is not prone to tailplane stall only if operated IAW AOM. It's still off the record but Q400 testing has shown that even if one somehow manages to get significant ice on the horizontal stabilizer, chances of tailplane stalling are very nearly zero as no test has pushed the tail near the brink of the stall.
It is possible that aerotunnel and flight testing did not cover every possible form of ice on the stabilizer so the manufacturer has cautiously decided not to pronounce its aeroplane tail stall free. However now there's a dilemma whether to tell the crews that they don't have to worry about the tailplane stall, and risk that the unlucky few who somehow manage to defy the laws of the probability and stall the tailplane don't know how to get out of it, or teach pilots to recover from tailplane stall and risk the procedure getting applied at inappropriate time. I don't envy those who have to make that call.
Thank you for pointing out the part in the FCOM, Excrab. It is very good advice but my speculation is that it was written as a general warning based on NASA's tests in twotter (BTW its configuration is as bad as it gets for tailplane stall; high wing coupled with low tail and powerful flaps). Stick force lighting is what happens as the tail with unpowered elevator approaches the stall but I can't see the way in which nearly stalled tail can fool Q400's pitch feel and trim unit into decreasing the stick pressure.
As for why did the crew do what proved to be their final undoing, I'll have to disappoint some of the readers: more often than not the accident investigations don't manage to get that deep. We still don't know what made captain of PH-BUF to attempt take off without clearance or captain of TC-GEN to put his thrust in the only faulty instrument on board. The investigation is finished when it can be said with high probability: "What happened is this-and-this and we have to do that-and-that to prevent it from recurring". And there's good chance we'll never be able to find out just why the stall warning recovery procedure applied by the crew was not only contrary to type specific one but also to every other known to mankind. My speculation is that it was both pilots' incapacitation brought on by the fatigue.