It was disscussed many times that in Ed all tests are action tests (even dmg tests), whenever you take an action and have to make a test...its that action Action Test (including resisting tests and so on).
Ofc there will be some strange things like do I take penalty from Taunt to resist poison?

Its GM call. As GM I would say not to pure TOU, but definitely to any Talent that helps resist it, as using talents is only possible when you can concentrate on your Personal Vision of your Discipline.
Penatlties from many uses of Terror dont stack, but that doesnt stop the Horror to use it again for a better result.
Terror was in previous editions a really terrible power, even more that it is now:
Ed3 and Ed classic:
"...If the test succeeds against any character, the affected character must make a successful Willpower Test against
the Horror’s Terror Step before he can perform any action against the Horror. Until the character makes a successful Willpower Test, he can take no other actions. His speech becomes a piteous wail, and any movements become random twitches."
So as you see Terror should be a Really Bad Thing. As written it should aplly. But you can do as you want.