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1) which beings are affected by the absence of light (partial, full, complete)? does the magical nature of the darkness make any difference (ethereal darkness)?

shadow hunters?
tasked spirits?
spirits?
cadaver men?
horror constructs?

2) can the caster of evil eye raise the difficulty number of lifting the curse by using willforce on his spell? (it explicitly says "the magician's willpower step", not "effect test".)

1) shadow hunters: do not need light to see.
2) tasked spirits Game Master Discretion. Generally those that summon a diunral creature such as Astral Mount need light to see. Whereas blood servitor would summon a nocturnal creature such as krilworm and would not need light to see (beyond that they have locate target). 3) Most spirits can use astral sight to navigate rather than relying on light.
4) Cadaver men: Game Master discretion, personally its no, they can generally sense the presence of living because of their hatred for them.
5) Horror constructs: Game Master discretion. Mostly no. I'd rule that some with a creature base like Black Mantis may need light to see. Nightwists would not need light to see, being constructs of pure darkness.

As for Evil Eye, no the caster cannot use Willforce to affect the Difficulty Number, as the spell's Effect is -5 penalty to all Action Tests, except trying to resist the spell. See Effect Tests p.141. Air Armor is not affected by Willforce in the spell example, as it has a set effect of +3 Physical Armor. Evil Eye is a spell with a set effect.

i think u misunderstood my question. i asked: if A casts evil eye on B, A using willforce for casting. Does B have to beat A's willpower step to break the effect next turn, or does B have to beat A's willforce step?

Willpower, caster can't use Willforce.
What slayride is hinting at is that the description does not mention the Effect Step for resistance, and Willforce can only be used to replace Willpower on Effect Tests. As the Effect line is not used, Willforce is out. You can't just put Willforce in anywhere in the spell where it says Willpower, just in the Effect line, and for resisting.
(There's other spells where you make a resistance against the Effect Step, and then, you may use Willforce, which is why the difference matters.)