in regards to the second question.
"imagine the atmosphere to be very cold, well below the temperature for which the altimeter has been calibrated. The air molecules have lost energy and therefore gravity is able pull them closer to the Earth. Under these conditions the air's density and pressure fall more rapidly with height. Now imagine the atmosphere to be warmed to a very high temperature, well above that for which the altimeter has been calibrated. The molecules have gained energy and can counter the force of gravity, so the change in density and pressure with height is less. If the altimeter is used under the two conditions it will experience, for the same true height change, a greater pressure change under the cold conditions than it will under the hot conditions. Because it converts pressure change into height change, the altimeter will register a greater height under the cold conditions than it will under the hot conditions even though the actual height ascended is the same." --from:
http://bubl.ac.uk/org/tacit/tac/tac43/theuseof.htm
Seems as though that should be right though. I'd love to hear some other comments on the subject though!