Again,
We have sworn testimony from a professional firefighting crew specifically trained to deal with potential issues specific to the Concorde...after all that's why they were there
who state (and this is multiple individuals) that the fire appeared to start prior to the estimated point of impact with the titanium strip.
Given both the training and specific reason they were present they meet the criteria for expert witnesses. This casts a measure of doubt on the debris being the actual cause of the fire.
Now lets go beyond that and look at other variables. While the asymmetrical engine thrust undoubtedly caused a significant portion of the drift the missing spacer has significant potential to add to that drift...potentially causing the impact with lighting etc...
While computer projections project that regardless of flight crew actions the wing/control elements would have burned through...we do not know if proper action by the FE would have allowed continued flight until that point and what the margin of error for the projection is. Regardless of it being 1% or .0000001% the FE's actions are what doomed the plane at that moment. The actual crash occurred when/where it did because of his inappropriate conduct.
The reality is that no one truly objective can say with certainty that the titanium strip caused the fire or that if the flight engineer acted correctly the plane had zero chance for a controlled crash landing that would have allowed for some survivors.