The smoke definately looks like the engine being stop cocked from flight power. I've seen that done by a slightly over-enthusiastic TRE and the effects were very similar.
I haven't flown a 350 for some time, but our TREs used to simulate a tail rotor control jam at high power by putting in enough foot and making us land without any pedal inputs.
A TR drive failure is the time we all become test pilots. Very few people know how any aircraft will behave without TR drive (even fewer survive the experience to tell the rest of us what to do) The designers will have a model of the idea and that is what goes in the AFM, and most of the idea will come from the textbooks.
I had a TR drive failure a long time ago and the AFM was totally wrong in what it thought should happen. Even in auto-rotation directional control was a hell of a lot less, and a lot harder to manage than I expected. Luckily we were fairly low and the impact was with a fair chunk of forward speed. However I could feel that we were on the edge of a severe spin, probably with catastrophic results.
I also wonder what the pilot's survival chances would have been with the new seats fitted to the 350 range. From the picture this aircraft had the old ones, the new ones would have crushed a long way down and maybe kept him clear of the roof and cyclic.