-- Saul's proposed defense of duress is kind of ju-jitsu genius, because it uses the strength of the government's case against it. To borrow a phrase from Saul, the government's case is that Saul was the Tom Hagen to Walter's Vito Corleone. It would show that Walter was unspeakably evil and Saul facilitated that. Well, the more evil that the evidence makes Walter look, the more believable it becomes that Walter forced Saul to do it. In such a trial, Marie's grieving widow testimony would help Saul -- it would show that Hank had no clue that Walter was Heisenberg until the very end; that Hank's medical bills were paid for out of drug money; that once Hank found out, Walter tried to blackmail him; and that when blackmail didn't work, Walter was present when his brother-in-law was murdered. Those facts would all bolster Saul's claim that Walter was a charismatic evil genius who forced him to participate.
I know a defense lawyer who represented a man who kidnapped and threatened his business partner, believing that the business partner was about to betray him. The defendant pleaded an insanity defense. The prosecutors kept emphasizing how the business partner had never betrayed or hurt the defendant, which the defense lawyer used against them to argue that only an insane man would believe that this business partner had done him wrong. The defense worked and the man was found not guilty by reason of insanity.
Saul would have run into problems with his defense, because duress requires the defendant to show that they went to the authorities to report the crime as soon as they were able. To use an example, if I hold you at gunpoint and order you to drive a car full of drugs to a Walmart parking lot, the defense of duress requires you to either call the police or drive to the police station as soon as you are no longer in immediate danger. Saul would have a difficult time arguing that he had no opportunity to contact the authorities during the 16 months he worked with Walter. But this would have given the government some big headaches.