All excellent points, and interesting about leather in general when drying it contracts tightly, so those notes for use make sense. There indeed must have been a number of processes known for cuir boulli, probably depending on materials and preferences, but the wax must have been a challenge.
A number of months ago we discussed the cuera used on the Spanish frontiers in the latter 18th century, and these seem to have been fashioned of multiple layers of rawhide, I think as many as 6 or 7. Therefore thier resilience as a defensive armor depended more on layering than the type of processing in cuir boulli. I am not sure if the various Indian tribes might have used leather hardened in these processes, though certainly rawhide was used in clothing etc.
Well said on the effects of heat on metal vs.leather in hot sun.....and I sure wouldn't want metal on in those conditions!!
All the best,