Lanloarn, I think you're missing something important in the very quote you're using.
QUOTE
A critter with Immunity has an enhanced resistance to a certain
type of attack or affliction. The critter gains an “Armor rating” equal to
twice its Magic against that damage. This Immunity Armor is treated
as “hardened” protection (see Hardened Armor above), meaning that
if the Damage Value does not exceed the Armor, then the attack automatically
does no damage. Additionally, this “armor rating” is added
to the damage resistance test as normal armor.
Immunity to Age: Some beings possess immunity to aging. These
beings neither age nor suffer the effects of aging.
Immunity to Normal Weapons: This immunity applies to all
weapons that are not magical (weapon foci, spells, adept or critter
powers). If the critter has the Allergy weakness, then the Immunity
does not apply against non-magical attacks made using the allergen.
"As normal armor" reads that for all intents and purposes the armor rating is treated like regular armor. Regular armor's rating is modified by all types of AP.
The modification of armor ratings takes place
prior to soaks as spelled out in combat and armor rules. The effect for Hardend armor creates a 100% soak pass fail test, the test for this occurs at the time damage is soaked. Whether or not we call it Hardend armor or something in air quotes, we are using that same effect and that effect is not changed in the description here. Therefor, modified armor values are used when calculating the effects of ITNW.
Seems to me like it's both backed up by it's (admittedly poorly written) language, and proper use of game mechanics. To treat it otherwise would require a deeper explanation in the description of ITNW. If the rules to not explicitly provide an exemption, all other rules apply as normal.
QUOTE
"Lasers are the exact same thing as tasers for this purpose, they reduce impact armor by one half. Spirit don't have impact armor, they have ITNW which is an "armor rating".
By this reasoning spirits receive no benefit from ITNW
at all. A gunshot is soaked with Ballistic armor, and a spirit has none, they have INTW = 2xF. Likewise, Lasers reduce impact armor, a spirit has ITNW = 2XF. If you are going to say it cannot be halved because it isn't the same type of armor then you should not be using it when the rules specifically call for the use of said armor. As such by your interpretation that they are not interchangeable, a spirit receives no armor to nearly all types of attacks.
Instead, ITNW is treated as regular armor, (both ballistic and impact) AS REGULAR ARMOR. The only difference is that if you don't exceed the modified armor rating the defender makes a full soak.