Can't they just hand it to you say "you enter your passphrase, but don't divulge it to us and then hand us the phone". In other words hinging the passphrase divulging to the 5th can backfire in that respect. It like saying we have a search warrant, you open the safe for us, it's fine if you keep the combination to yourself, we just need to get inside.

No, that's pretty absurd. It's not specifically about the act of speaking. It's the act of incriminating yourself.

But that's all beyond the point, anyways. If they did hand you your phone and said, "enter your passphrase," you can just say, "I don't remember it." They can throw a fit and put more heat on you in various ways, but until they resort to torturing you or they develop mind-reading technology, there's not much they can do at that point until the case reaches a judge.

That's not to say "I don't remember" is a sound, blanket defense. But it's sufficient for demonstrating that these dynamics all depend on willing participants which is partially why these laws are designed the way they are.

The act of unlocking it can incriminate you. It's ~proof that you have control of the device beyond what they already knew.