A) No, you will not save energy reaching a lower boil temp at altitude.
B)You have less O2 available, thus, you will have to "lean out" your fuel to equal the avialable O2. Initially, you will burn less fuel, however, you will need to burn it longer because the BTU will be lower.
C) Is 185 less efficient than 212 at sea level? You will end up having to burn/cook longer to maximize available heat, so I would say "Yes".
FYI: The physics law that applies to this whole process is Boyles Law (explains the effect of atmospheric pressure changes)
