Yes, but the rule of law in the US is mostly intact, so refusing to provide information or actively facilitate a search will not have that result. Lying to the officer can.

Yes, if they actually find probable cause to charge you with a crime, then they’ll let you enter but arrest you and put you in jail. Refusing to unlock your devices does not qualify, though.

Sure, but without a criminal charge there's the right of habeas corpus. (Provided you have someone who will get the legal process started for you, I guess.)

OP was talking about being forced to unlock devices.

Is there evidence of US citizens going to prison for the act of refusing to unlock a device at the border?

No, none that has been presented.