[const_cast]

We coerce and force people to relinquish passwords here in the US all the time. If you go through anything related to the border, that's run-of-the-mill. They don't even need a warrant for most of it, your fingerprint can be coerced out of you legally.

[OutOfHere]

If a US citizen does not unlock the phone when entering the US, the phone can be seized while they send it to the lab to unlock it, and that's about it.

If the phone gets unlocked, the border police cannot coerce unlocking specific apps at all, although a judge can. The border police may however choose to lie about it.