Dab
The AAIB report, as I read, infers the Fenestron was still able to be turned by the No 2 engine FPT accepting however that the Fenestron detached during the crash sequence. I think this rules out Fenestron related theories!!
No they haven't and no it doesn't.
You may have read it that way, but what the report says is;
"Initial assessment provided no evidence of major mechanical disruption of either engine and indicated that the main rotor gearbox was capable of providing drive from the No 2 engine power turbine to the main rotor and to the fenestron drive shaft."
Which means that drive was capable as far as the trds. Therefore everything rearward of and including the flexi coupling connecting the TRDS to the MRGB is still in play as possibly contributing to the incident.