Good summary, the only difference between this and the house key example is that you can "forget" the password. When you "forget" there is nothing they can do besides say you are unstable and hold you in contempt of court.
True, and shredding a key would be no different than shredding evidence. You could be charged with destroying evidence but not the crime the evidence supported.
I dot think you'd be charged with destroying evidence, because that would be assuming that there would be evidence. Instead, I suspect you'd be charged with contempt or obstruction of justice, or something along those lines. Otherwise, yes, I think you're right.
Under civil law, a finding that evidence was destroyed implies a finding of the fact that the destroyed evidence would have supported. I'm not sure how this would affect a criminal law proceeding, though I'm reasonably the prosecutor would be allowed to introduce that fact to prove guilt. (Ultimately it would be up to the jury to decide.)
Plaintiff: "There was a dead-body in the toilet, but the defendant has been regularly flushing, thereby destroying the evidence and proving his guilt."
I was referring specifically to civil matters, not criminal ones. A relevant example would be a claim that D defrauded P; a finding that D intentionally destroyed evidence that would prove the fraud could be used to support a finding of fact that the fraud occurred.
