Phil gollin,
See my post 1689 of 2 September. What follows is speculation, but consider this. Boeing, RR and the AAIB realized that there was not enough water to produce a sufficient quantity of ice to restrict that fuel to the engines and concluded that other factors were in play. They may suspect or have discovered after carrying out CFD modelling, that the fuel they are analyzing has exhibited non-Newtonian properties at certain temperatures. Examples of non-Newtonian fluids (also known as complex fluids) are blood, paint, egg white, and many other sticky fluids. The nature of these complex fluids is known, but their properties and the changes they undergo are difficult to predict when it comes to transporting them in pipes. Not too much is known about the effect of temperature, the onset of turbulent flow, and the propensity for agglutination of these complex fluids.
Regular jet fuel is a Newtonian fluid, as is water and other simple small molecule fluids. Perhaps the “exceeded spec.” fuel with a superior freezing point, displays non-Newtonian properties at certain temperatures depending on the architecture of the fuel delivery system? But this is idle speculation and largely a waste of time (as I think you said before) until we have more facts.