Jack Lifton in [0] adds the perspective that rare earth processing involves a large amount of institutional knowledge, that would take time for the US to reacquire, since we forget everything when we stopped mining many decades ago. While in the interim, China has spent its time as the world's rare earth monopoly optimizing the chemical processes, widening their moat. It's like the manufacturing question, where's it about institutional knowledge and industrial ecosystems—nothing at all like oil/gas where it's solely about where the minerals are.
This is exactly why Greenland could be needed to compete with Chinese production. With Greenland an offshore legal gray area like Guantanamo, you just need to add work camps stuffed with "Illegal Immigrants" and other Enemies of the State. In fact, why have prisoners on shore at all when it is more profitable to have them work offshore? With foreign news sources banned because of False News, the huddled masses will be educated that this is a good thing.
You can do it all via Canadian waters and you can't do it at all without using Canadian waters so no need to even consider greenland as they don't have any waters that would speed up the transit.