You are right that the police can't force you to talk and that a judge can't if you are on trial.
However a judge could force you to testify in someone else's case. You can't broadly plead the 5th in that case unless the testimony is incriminating. Let's say you saw a murder happen at a hotel you were at while cheating with your wife. A judge is allowed to force you to answer the question.
Yes.Infact, if you conceal the information about the crime and the crime happens to be felony, you will be charged for "misprision of felony."
The federal definition of misprision requires that, “(1) the principal committed and completed the felony alleged; (2) the defendant had knowledge of the fact; (3) the defendant failed to notify the authorities; and (4) the defendant took affirmative steps to conceal the crime of the principal.” See United States v. Baumgartner (6th Cir. Sept. 24, 2014)
However a judge could force you to testify in someone else's case. You can't broadly plead the 5th in that case unless the testimony is incriminating. Let's say you saw a murder happen at a hotel you were at while cheating with your wife. A judge is allowed to force you to answer the question.