Several Points
1. The "misfiring engine" is often reported by supposedly reputable eye-witnesses, but can usually be put down to "blade slap" or something similar. Beware eye-witness statements that give a conclusion; a little knowledge is a dangerous thing.
2. There has been several accidents wher Nr has reduced to zero before impact. It is a natural reaction to prevent impact by raising the collective as much as possible. If done even slightly too early, Nr reduces to zero and the aircraft assume brick-like aerodynamics.
3. If the AAIB say that the drivetrain and engines were capable, then they are saying it was all serviceable before impact.
4. There seem to be quite a few similarities to the HMS Portland Lynx Accident in 2006 (I think). Zero Nr, Vertical descent etc. I'm sure the AAIB are comparing.