Lyman:
By use of the term "shaft failure" I mean the functional disconnect of the IPC from the IPT, regardless of the specific location of the break. The IPT drive arm is one component of the shafting. You can theorize all day long about vibration, shaft harmonics, etc. but these are merely possible logical routes to the ultimate shaft disconnect.
Good design practice assumes that the shaft will, despite all your efforts, suffer a failure someday; and it's the designer's job to mitigate the failure.
And yes, I know that R-R N2 sensing is at the compressor end, so there's no hard data from the IPT itself if a disconnect happens. But the observed N2 spooldown is non-characteristic when the other rotors are still up to speed, and should be interpreted as the signature of a shaft failure, IMHO.