The Fan, LPT rigid coupling is within the IPT Shaft, and I see no oiling capability. How would an oiler penetrate the IP Shaft? My assumption is perhaps grease, or no lube for the Long Shaft. The LPT Shaft fits within its forward mate, so the oiling is, what? I think fire, of course, DevX has said so. However, Fire alone? The fire may have plasticized the Shaft, as you say, but I think failing an aft movement, there may have been no "contact" with the Stator (LP). The leak is known to have existed prior to failure, as has the Spline Wear. So whether "Acceptable" or not, the utter failure, imo, has to do with oil starvation after a high effort climb, high temps, and massive vibration.
Without Oil, all manner of chaos would ensue. Consider "Friction Stir". If the bearings, in thrust and carriage, failed utterly, a very quick aft movement, a Disc "Burst" and seizure of the Shaft? Without the IPT all combustion is free to migrate forward, to exit the Low Pressure Stator vanes, and foul the cowling? If the IPT Hub was plastic, perhaps seizure caused the separation, Overspeed is unlikely if the bearings were dry?