Lyman:
...any separation of the Power Arm from the wheel eliminates the cvarriage for the Turbine Disc, via separation, The Disc has no structure left, it is not as if the wheel is inboard, and merely spins on, without bearings.
I assure you that the intact disc (minus the drive arm flange) is only unsupported for a few milliseconds, as the gas pressure drives it aft to a false bearing surface in the LPT inner duct. Now it is once again supported against a fairly low-friction surface, akin to the thrust bearing on your car's crankshaft against which your clutch reacts.
Without a significant torque load (normally the IPC, which is now un-driven and winding down), the IPT has no chance to do anything other than spin up to destruction. Over the decades, I have seen the results of like turbine disconnects; the sequence is always fairly similar, and the results never pretty.
You seem to keep referring to the IP main shaft, but is there any mention of a pertinent shaft defect in this accident report?