score:3
"Does an act being intrinsically evil ensure culpability?" No.
A person forced to commit an intrinsic evil does not sin because sin requires consent of the will; "every sin is volutary" (St. Thomas Aquinas, Summa Theologica I-II q. 80 a. 1 co.).
cf. PrΓΌmmer, O.P., Handbook of Moral Theology on human acts.