1) In NM freeze and chill durations are reduced by 50%, and in Hell they are reduced by 75%. In Hell, instead of freezing for 4 seconds as stated on the skill, you would freeze for 1 second. The
ArreatSummit has some other effects of changing the difficulty level.
2) In HU you can use all 3 cold armor skills at the same time, and you can cast FrozenArmor on other players and pets.
3) FrozenArmor should freeze enemies only if they have actually done melee damage to you. If you are invulnerable to damage via DR/MDR or if the enemy miss due to your high defence or block, then they will not be frozen. It was recently reported and confirmed that FArmor is not freezing anything currently, perhaps due to some typo in skills.txt. ShiverArmor will counter-attack anything that attacks you in melee, regardless of whether it hurt you or not. ShiverArmor does not freeze, it only chills.
5) The Cold chill durations of ShiverArmor and ChillingArmor start at 4 seconds and gain 1 second per level. The chill effect might not be noticeable since a lot of creatures have a property that lets them reduce the effect of chill (similar to how monsters have a property that reduces the effect of leach).