A couple minor questions. When replacing Intelligence or Willpower by your Insanity/Corruption do you use your Insanity/Corruption to calculate your Int/Will bonus for effects that use it? Related question, If you have unnatural Int (x2) does your Insanity score gain the benefits when rolled in place of Int?

I would say yes to the former, but only if you choose to replace the statistic for the purposes of that Test.

As to the latter, no.

The Trait in question, for those curious:

UNHOLY INSIGHT (TRAIT)
Your mind has been touched by something vast and
damning. Whether through soul-scarring clarity or the
vile artifice of an insane mind, you hold secrets that no
human being should ever possess.
Whenever an Explorer with this trait attempts a
Forbidden Lore Test, he can choose to gain a +10 bonus
on the test, as the twisted architecture of his mind sees
patterns that make little sense and come to conclusions
that are disturbingly precise. However, each time the
Explorer gains this bonus, he gains 1 Insanity or 1
Corruption Point (whichever is appropriate). Additionally,
when attempting any Intelligence or Willpower test, the
Explorer may instead substitute his Corruption Point or
Insanity Point total for those Characteristics.

Hello !

Where is this Trait please ?

Into the Storm, page 105.