Cattletruck, I've no experience with FADEC helos, but in other types with hydromechanical FCUs (S61 & BV234) an engine surging and banging as you describe can cause the most confusing signals as the good engine tries to compensate. You can get N1 & EGT see-sawing violently, one engine up, one down and vv which can make diagnosis of the bad one very tricky unless you rationalise it correctly.
However, in that situation you know you are in danger of selecting the wrong one and act accordingly with great care.
I'm with TC in this, and of the opinion that double engine failure in a twin is regarded so unlikely it builds a mindset that it "can't" happen - and the level of confusion and sheer disbelief when a donk stops following a red fuel warning (surmising here) when there is clear indication of fuel in tanks would leave anyone in a state of momentary stasis. Follow that with a second engine failure, perhaps when beginning to deal with the first one and the couple of seconds of "Oh no! Oh Christ! Oh sh!t! Oh dear! might well be enough to lose the Nr to a point beyond recovery. Once that has happened (at what, 80%Nr or so?) the drag on the stalled blades plus the induced airflow angle from the increased r.o.d. would slow them very fast indeed.
Two things are certain. Engines and rotor stopped.
This can only happen in certain ways. Power is either lost by malfunction or by pilot action. Can't be anything else.
Nr is lost by insufficient power to drive the transmission, so either by power loss and too much pitch retained (because if pitch is reduced autorotation occurs) or the power was robbed by a seizure etc - and we believe it wasn't the latter because the drivetrain appears able to turn and MGB intact.
It's hard to see an alternative to "something" causing a power loss but autorotation not being established leading to catastrophic Nr loss and subsequent loss of control. With the best will in the world it is equally hard to see what can cause such a loss of Nr except the pilot not reducing pitch enough, or quickly enough, or not keeping it reduced.
The questions to answer are "what?" and "Why?"