With reference to forgetting to add power. I've seen it in the simulator, usually when the pilot is distracted by something. Sometimes it's not so much forgetting to add power, but not adding enough power for the situation (one engine with gear and flap out for example) and then not keeping their scan up so they don't notice the degrading performance (e.g, distracted by performing a circling manoeuvre in bad weather with an engine out.) I'm not saying it's what's happened here but the concept is not far fetched.
I also saw it a lot from a pilot who was new to the aeroplane and new to turboprops. He wasn't used to the lack of aural speed and power cues compared to the piston engine aircraft he'd been flying. But that was when he was just doing his endorsement so his hours on type were in single digits, far less than the crew in this accident, and he soon learned to bring the power levers up when leveling off.