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.