Canada

Theoretically NATO runs on consensus decisionmaking, so theoretically having even one member not aligned with the rest this would implode NATO.

Realistically, this is not a limiting factor because when consensus cannot be reached, there is nothing stopping the supermajority of members from reaching their own consensus and acting accordingly. Even a small minority is always free to do this, provided as the US proved with the infamous "Coalition of the willing".

If the US, or any member, turns against NATO, it may be technically impossible to eject them or for it to continue to function as an official organization. However, the organization is not the valuable part of NATO. The members are the valuable part of NATO. Losing a very valuable member like the US would be a blow, but there are many other valuable members in NATO and they would still form a strong collective even without the US. There is no reason they couldn't immediately reform as "NATO 2.0 with blackjack and hookers" and exactly the same organizational structure, and the problematic member would not be invited to join, and this would be functionally almost exactly equivalent to NATO ejecting the problematic member, so we could probably just keep calling it NATO and say they ejected the problematic member.

Trump will not attack Denmark. He will "merely" assert that the existing US base on Greenland is sovereign US territory. It will not be an attack, it will be a "do something bitch".