A catastrophic MGB failure would not even cause the Blades to stop turning....but it would make them and the gearbox depart the aircraft in all likelihood due to the massive forces created by that event.
Harken back to a North Sea crash a while back.
A more gradual failure of some sort....perhaps they might slow....but in all likelihood both Engines would still be applying power to the MGB.
As to the AAIB comment.....All Descents are Vertical by definition....and we should not put too much credence in that very short, vague, comment as there was no additional discussion of the manner or pitch of that Descent.
The AAIB will have a Preliminary Report of some sort for public consumption within a very short period of time due to the massive public interest in this tragedy.
What they say in that will begin to point us in the direction they will be going in their investigation.