A couple of comments. Police are prevented from illegal coercing, which generally means they can't physically abuse someone, threaten their harm against them or their family, deny them food, sleep, etc. What the Wisconsin police did is fine. The court properly allowed it in and it was for the jury to decide how much weight to give it. The investigator was hired by Dassey's attorney, which is all the more odd. Len simply decided his guy was guilty, didn't want to try the case and was going to beat that kid into cooperating and pleading. If (if) I had to defend it, I would say that Len knew he'd get convicted regardless after that 1st confession and was trying to look out for his best interest and get him a deal vs. what he got.
I'm telling you, there is a lot of good people who when put on a jury like that will not have the courage to go against the police and the district attorney. They fear what their neighbors will say when they get off that jury, they fear the chance that the accused did it (and will do it again) and they make a mistake and let him go.