In all areas where you are affected by Yuletide, the armor level on your character's armor has no relevance, as all snowball skills ignore armor.
"You may only attack with attack skills" means that you cannot use your weapon's ordinary auto-attacks. When attempting to auto-attack, you will still move towards the target until you are within range for your weapon's attack, e.g. adjacent when wielding melee weapons.
Yuletide is not applied to Jack Frost or the Grentches and Snowmen found in other areas.
The +4 Energy regeneration replaces natural Energy regeneration and Energy regeneration from equipment instead of adding to it.