Philipat:
A dual fmc failure (as one of many possible failures following an MEC fire) would mean a diversion in alternate nav. This has huge potential for incorrect positional entries, particularly under stress of (possibly) cockpit smoke, depressurisation, etc. not to mention unfamiliarity with how to do it. It could explain why the aircraft flew in such a random way, my point being that the erroneous waypoints would have been loaded at the point of diversion or immediately after, not later.
Direct Ortac:
I referred to that AAIB report in my first post. The extent of damage after only 12 minutes was alarming.
According to that AAIB report, there IS smoke detection in the MEC but all it does is place a decrete in the FDR. All that will alert the crew is an EQUIP COOLING OVBD advisory, which is treated almost as a 'nuisance' in the NNC. There is NO EICAS MEC smoke warning on passenger B777s. There is NO fire warning or suppression in the MEC. If that has changed I'm sure someone will enlighten me.
I'm not going to labour the point any further. The scenario I suggest, or something similar, is just one way that all the dots can be joined up between TOC and the SIO. Hopefully we will find what happened soon.