The fact that they don't need a warrant is irrelevant when there are technological measures which require your cooperation to make it happen. The question is, what happens if you don't cooperate?

For fingerprint authentication, I believe it's been established that they can hold you until you cooperate.

For passwords, they can't. "The second question is, can they detain you until you hand over your passcode? They probably have some power to detain you for a little bit — maybe for an hour or two — not forever. They can't hold at the border and say, ‘Alright, you're living here until you hand over your passcode.’"

Then the question is whether they can follow up with some sort of punishment after the fact if you continue to refuse. According to that article, it hasn't been established yet, but I'm confident that it would come out in my favor, and furthermore that they would think similarly and not even try it.

As far as I can figure, if your phone is powered off and requires a passcode to run, the only thing they can do is confiscate your device. They cannot bar a US citizen from entry into the country - even if you have no passport or identification, they can only fine you for entering without it.

If they confiscate your device I'm not sure how long they can keep it off the top of my head, but I would operate under the assumption that you're not getting it back. There are legal remedies for this sort of warrantless seizure, but I'm guessing that there's a mechanism through which they could charge the phone itself with a crime in order to circumvent the Fourth Amendment, a'la asset forfeiture.

Right, that's my understanding as well. I'd lose the device but retain my freedom and (whatever shreds remain of) privacy, which is an acceptable tradeoff to me given the relatively low odds of this happening.

The Fourth Amendment is interpreted quite differently in border zones; the side of the Government is generally weighed significantly heavier "because terrorism".

You can remotely wipe most phones today, no?

which requires the phone to connect to the internet, which i doubt the TSA/DHS/CBP will let happen after confiscating your phone

They can have my iPhone. I can get another and restore from iCloud. Good luck to them to do anything useful with it. Sure I might have to wait for a bit but they have to let me in.

you have to give your fingerprint in order to pass US immigration, surely the correct answer is "I have already given you the password" .... and then let them figure out how to extract it from their system and apply it to your phone ....