(IANAL.) Employment agreements generally are not enforceable when they conflict with the law, AFAIK. The security guard might be required to never open the door, but when a police officer or court gives them a lawful order to open it, they open it or go to jail. I doubt they could be fired for opening it in that circumstance; your employer cannot require to you violate the law.

Another way of looking at it: You can write whatever you want on paper and sign it, but that doesn't make it enforceable. You could sign a document that makes you a slave or puts up your children as collateral for your ISP bill, but those agreements would be scrap paper (or evidence for the prosecutor).

Not scrap. They do evidence an intention to pay. They are an acknowledgement of the debt. The individual clauses may be struck as illegal, but the documemt and signatures are still a thing.

Which is fine, since you're a US citizen and can't be denied entrance. They're going to confiscate your devices, though.

Can LPR’s be denied entrance for refusing to unlock?

No, an LPR can only be denied entrance if:

(a) They are regarded as seeking admission and they are found inadmissible. Refusing to unlock their device will not make an LPR regarded as seeking admission as it’s not a reason listed in INA 101 (a)(13)(C): https://www.uscis.gov/ilink/docView/SLB/HTML/SLB/0-0-0-1/0-0...

(b) They are deportable. Simply refusing to unlock a device in itself will not make someone deportable as it’s not listed in INA 237 “General Classes of Deportable Aliens”: https://www.uscis.gov/ilink/docView/SLB/HTML/SLB/0-0-0-1/0-0...

Probably not, since they also can't be denied entrance by executive order. But I'm not a lawyer!

I believe an LPR could be paroled, pending a hearing with an immigration judge.

Saying that, the only people who cannot be denied entry to the US are indeed USC.