1. (A) No. All crimes require an “actus reus” (an act). The act must be a voluntary one. Here, the actus reus requirement is not satisfied, because Seth's act was not voluntary; he was, in effect, Cain's weapon. Since there was no voluntary act on Seth's part, he cannot be criminally liable.
(B) Yes. Here, Seth's actual act was voluntary, even if he wouldn't have done it “but for” Cain's threat. (Note that Seth may be able to defend against criminal charges due to duress, discussed in Chap. 4 (II), although it's doubtful he'd win because duress is generally not available for homicide offenses.)
2. No. In order to be criminal, an act must be voluntary – that is, the act must have been committed under the actor's will and control. Where the act is the result of an epileptic seizure, it is not voluntary and thus no criminal liability will attach. (However, an epileptic might become criminally liable for putting himself in a position where his potential loss of muscle control is likely to cause serious damage, e.g., by driving a car. Here, the actus reus would be the reckless act of driving while knowingly subject to seizures.)