Or loads a dummy profile and deletes other profiles without giving away what is happening.

I had this feature on a Xiaomi phone that i had a few years ago. I could even set up the fingerprint reader to open a different profile depending on which finger i used.

Dont think that would be as secure. Once a phone has been unlocked after reboot, even if it has been locked again, the operating system and ram can be accessed quite easily with the correct tools and border controls have those tools.

Even a Graphene phone is only properly secure before first unlock after reboot. So i would not personally rely on allowing acess to any profile. I would assume its possible, (probably trivial) to gain complete access from there.

Any duress pin use should be covered under a person's right to avoid self incrimination.

If the phone is owned by your employer and they are the investigation target, then you might be on dodgy ground.

Alternately, a user profile that isn't displayed anywhere, but accessible only through a specific PIN, maybe. Not sure how you'd explain the storage space either option used, however.

Nothing is needed. Erase your shit before you travel in or out of the US.

You can already have multiple independent users in graphene (maybe in stock android too idk). You have to switch them before the possible search though.