Keg,
I think you've explained this well, except the bit about OAT.
In fact, OAT has nothing to do with the relationship between IAS and Mach at a given FL. It probably isn't obvious on your 767, since your Econ climb/cruise is slightly different every day, and therefore the crossover FL is probably also different.
On simpler aircraft which don't have an Econ function in the FMS, which for example climb at a fixed 250KIAS/M0.72, the crossover (when 250KIAS equals M0.72) ALWAYS occurs at FL336, regardless of ISA deviation.
This stumps many since they are used to the variation in TAS for a given IAS when the ISA changes.
Cheers,
VI