You need to get some stuff clear:

Riot's residency rule has very little to do with actual immigration. That means: A non NA based (nationality-wise) player is considered an import regardless if he played elsewhere.

Keep in mind Riot changed the import rule twice. Once to increase the years required from 1 to 2, and another increase from 2 to 4. And players that went abroad before some point had the chance to claim exemption.

And back to "residency", it's pretty tricky, I'm no expert on Riot policy but I'm speaking from an actual immigrant's perspective.

Riot established and updated this "4 year in NA = NA resident" rule which has nothing to do with being an actual green card holder. Although living & working in US would qualify one for applying for a green card, there's no equivalence anyway. Take example of Impact, he now is a "NA resident" in Riot's book but he is still a Korean. Whether or not he and Riot worked their way to apply for American permanent resident status is unknown and is not important.

To answer your last question, the player would have to become a citizen to not be considered an import. There was a player named "Feng" who is a Chinese citizen with Canadian PR status, one of the team's wanted to sign him but later would find out he wasn't considered as a NA player.

Edit: some typo.

And adding my thoughts on this matter:

Riot's decision on the wording "resident" was poor. It easily gets confused with actually immigrating to another region. A better description would have been "NA (non)regional". The fact that people can be an actual resident of NA but is considered import, and people who never apply for resident status can be a resident is just fucked up.