Old Engineer
What I am thinking is that if we have an oil fire, we have to look for a source of ignition. Talk about oil flash point (tech board) is all well and good, but the fire point is another, where a pool of oil under an ignited flash will burn without self-extinguishing. That's a bit higher, but the auto-ignition point is substantially higher--404 degrees C in the recommended Mobil Jet Oil II (as shipped from the factory, a highly-inhibited ester synthetic). There seems to be some discussion as to a change in oil in some of the QF32 engines.
I think the fire occurred in the chamber that is part of the frame located just forward of the IPT rotor. In a two spool engine, this frame would be best described as a turbine mid-frame. It is a very troublesome and complex component because of its location and the temperature gradients (stresses) that occur on acceleration/deceleration and resulting fatigue. On early two spool, high by-pass engines, this mid-frame was present, but on newer engines it has been eliminated by moving the bearings and bearing structures forward towards the compressor rear frame.
If you look at the Trent 900 cutaway, the frame (a multi-piece fabrication) consists of an outer ring (casing), airfoil like struts in the hot gas path (could be radial or could be tangential) connected to a highly formed and shaped thin wall 360º plenum chamber which supports the bearing system at the inside diameter of the frame. I would suspect each of the major components are of different materials selected for temperature, strength and formability considerations. I would believe the interior of the the struts and plenum are cooled from air drawn off the compressor to cool the oil lines that pass through the struts and plenum which feed the bearings and drain oil from the sump. Although not shown in the cut-away, I think the stub pipe in question is located in this plenum. This is a very hot area of the engine.
The gas path temperature entering this frame would be somewhere around 1800℉ (982ºC) or higher. I say this because the IPT rotor blades, which are solid (non-aircooled) are made of a single crystal high temperature superalloy.
Now if the stub pipe ruptures as they say it did, the oil pours out into the plenum chamber, no doubt causing a change in cavity pressure and perhaps overcoming the cooling air being provided. If the pressure becomes great enough, a failure could result in the plenum itself or the connection of it to the the airfoil struts resulting in a flash fire right next to the IPT rotor disc.
The heat generated increases the temperature of the disc itself, and it wants to stretch starting at the bore giving way to the failure of the power drive arm at the shaft flange 580 bolt holes resulting in the capability to begin to overspeed and move rearward.
That is my theory on how the fire ignition started.