The rule of thumb I have for correcting the indicated altitude for non-standard temperatures is: 4' per thousand for each degree C the temperature varies from ISA. Example: At 5,000' indicated OAT is 0. Thats 5 degrees less than ISA. So 5(thousand)X 4 = 20'. 20(-5) = -100'. Actual altitude 5,000 - 100 = 4,900'.
Since the difference is more significant at higher altitudes I read that some approach charts at higher altitude airports make adjustments in the MDA for very cold temperatures.
No, I never did find out why the QNH settings in the mountains around here always seem to be higher. Maybe it is just a local trend.