bearfoil
The seperation, Disc from Arm, would not be in any way orderly, and an imbalance (a certainty) would impart an elliptical and irregular orbit to the Wheel.
The problem I see with this is that it would completely wipe out the bearing structure below it and because the IP shaft is so close the the LP shaft, we would see rubbing or damage to the LP shaft. Based on the photos of the pulled LPT module (ASTB Report) there isn't any damage to the LPT shaft.
My drawings show the IPT Blades (Roots) as below ('within the perimeter') of the Stator Vane "Platform". "Cookie Cutter"** shear off of the Roots would force the Blades through and out the front of the fir tree dovetails. Given the "Shake" of the disc (wobble) in its new orbit, the roots would endure a constant force forward, while being mercilessly vibrated, a suitable explanation for their loss from the Wheel??
The recovered portion of the IPT disc (Figure 14 ASTB Report) does not show any damage to the disc posts between the dovetail slots and two of the turbine blade dovetails ("roots") that stayed in the slots look remarkably in good shape. That is why I think the contact point was just above the blade platform (airfoil) or maybe at the blade platform. If it was more toward engine centerline, we would see rubbing of the disc posts.
Now as to Boom, a Compressor Stall is possible, but so is explosive loss of LPT Drum (Gas) contents. A loss of this highly energetic gas forward and through the lost dynamic seal of the IPT would explain also the rupture of the case, pehaps in concert with exit of high energy debris.
The way I see it, we are talking about what happened in between 2-3 seconds of time. P30 collapse = compressor stall + bang. This occurred ~3 seconds or less before disc rupture. Now I also think that the IPT disc rupture and the casing ruptures (both IPT and LPT Stage 1 Nozzle + the nozzles) releasing gas pressure would be one bang, all happening within milliseconds of one another.
For some reason, I think the fuel wasn't completely cutoff, but reduced, perhaps because the ECC was programed to recognize a P30 drop (compressor stall) and to provide for rapid recovery (air relight) or the need to restart the engine completely (air restart), but I don't know this for sure. But I think the ECC didn't recognize what else may have happened. This fuel flow could have added to a fire that was subsequently put out by engine shutdown and by the release of the fire extinguishing bottles.
Just some thoughts...