Nintendo would see the sky fall down before they would do something like this, because unlike idSoftware, who release good work, make their money, and move on, Nintendo would re-release the same stuff 5 times, except it's always more expensive the next time
It would literally not hurt them to open source anything <= 3DS now after the eshop closure. The act of doing that and everything that would come of it (e.g. rom hacks) would actually be a pretty good PR move imo and maybe they'd even find new developers, acquainted with their code, that could be hired by them so they have developers that actually know how to develop good and optimized games.
Note that this is only the game code that allows people to develop mods, and not engine code that allows people to do ports. The re-release engine code is still proprietary.
They did the same with Quake 1 re-release game code
The license doesn’t really matter, unless you mean it should be included in the kernel. If it was another permissive license, you could also port it to Linux. I have no clue if it’s actually possible to port it to Linux natively, but I would guess that you can