FluidFlow (Ian): Welcome to the workroom!
I think we are all hoping for successful search and then recovery of the FDR and CVR. The sequence of ACARS messages, however, allows development of plausible scenarios and cause theories. The pitot disagree message has already effected industry changes (Thales to Goodrich).
Dean's 1979 paper on "Atmospheric Effects on the Speed of Sound" linked by Mr Optimistic points to the influence of water vapor at various temperatures in an ideal gas (see his Table 4) and talks about the heat capacity ratio Cp/Cv you mention. Seems the deltas are pretty fine, though...
Two thoughts:
1. Your proposition could be mathematically modeled if we knew the "standard altitude parameter" and the "credibility" test boundaries used by the TCAS software and mentioned by BEA to see if G can decrease sufficiently to increase M sufficiently to trigger an altitude calculation error and the ACARS warning message. Fluid dynamics, I've heard, is the trickiest of the physics disciplines, and the math is way beyond my own pay grade, but with the TCAS source code in hand I should think your what-if question could be ruled in/ruled out.
2. According to mm43, the WRN messages are sent in real time. Reciept of the TCAS NAV message follows the AUTO FLT AP OFF by 44 seconds. In your proposition, the TCAS NAV message, if the trigger, likely would have preceeded the other WRN messages.
GB