1) In regards to half/double/quarter/quadruple-damage from <X>, you would -always- take those, regardless of your armor value.
2) If you're immune to a carrier, you would take damage until that damage would hit your body, at which point you'd call no effect.
Example!
If John the Fighter has 10 Armor, and an enemy is swinging for 8 Life, he'd take the first hit (reducing him to 2 Armor), and then he'd call No Effect on the second hit, because John's immune to Life (being...alive).