I’m wondering if use of the Vuichard recovery technique could have induced a mast bump.
When I used to teach VRS sometimes if you were “lucky” to find it she would sink very quickly, enough to give that sinking feeling in the stomach accompanied with zero or low g. With the power been low there was only a small amount of pedal used to keep the nose straight.
Using full pedal for the side slip on the Vuichard technique and if coupled with low/zero g might be the factor that introduces the roll, especially if the disk is unstable due to VRS.
There is a photo out there where you can clearly see the shear is about where the blade would strike. I assume that would rule out gearbox issue.