A couple of comments
I've read most of this thread, but didn't see anyone addressing the below, apologies if this is redundant.
Night time Autos (especially when you're not expecting it) are very difficult, even when you practice regularly. Very hard to push the collective down, to make yourself go down faster at night into an unknown abyss.
I am aware of a couple of accidents with EC135's involving inadvertent movement of one or more throttles out of the fly position and into manual. This event manifested itself with subsequent loss of control and ground impact, two of which were at night. One of the accidents had video which showed one of the engines spitting sparks out as it oversped and relit, subsequently taking out the hydraulics.
I'm curious if Eurocopter ever moved the Fadec auto buttons from the overhead position to the panel?
Fuel contamination with water could be a possible scenario, other types of contamination would illuminate the airframe fuel filter light, assuming it had one.
Any sort of gearbox problem, main or tail usually involves temperature first, followed by metal chips all of which should show up on the annunciator panel.